You’re 18. Get a job and an apartment and move out.

Training in what?

I suppose a good reason would suffice other than just that you don’t want to go?
Have an alternative place in mind?
Find out why they want to send you?
Is it because they are going on a trip themselves and need to know that you are OK and in a safe place?
Or is it because they want to get you used to being away from them to get you used to independence?
Communicate WITH them not at them and tell them concrete reasons of why you don’t want to go and what reasonable alternatives that YOU may come up with..like staying with relatives or friends ?

Echo: “Training in what?”

Talk to your parents. Ask them to explain why it is important for you to do the training. Listen to what they say. If you listen to them then they might be more open to listen to your thinking.
You are very lucky to have parents that care about you.
Good luck!

Training for what?

Why don’t you want to go?

What are you going to do instead?

Are you graduating high school this year and are you going to college after you graduate high school?

First off…Welcome to Fluther! As a parent of adult children, I have had similar interactions as those you are going through. My job as a parent was always to help make my children the best adults they could be. And as a good parent, you always want the best for your kids. The balancing act is to know when to step away and let the kids be adults.
At 18, you are legally an adult. I don’t know if you have let your parents know that you are ready to lead your own life. Please note…leading your own life is not the same as being out on your own. It could be the same, but isn’t necessarily. I also don’t know what your relationship with your parents has been thus far in your life. Have they always dictated to you what you will and won’t do? Have they asked what you wanted? Have you voiced opinions? I’m guessing you have always pretty much done whatever they asked of you without pushing back. Not saying that as a bad thing, just an observation. The fact they are trying to dictate 6 weeks of your life at age 18 tells me they have established that pattern.
So you have several options. Option 1: Do what they want. You would give up 6 weeks of your summer and would get training in whatever they are asking you to get trained in. Option 2: Open rebellion. This is you planting your feet solidly in the dirt and telling them in no uncertain terms that you are done doing whatever they want you to do. Option 3: Sit them down and talk. This will entail you having some clue as to what you want to do. All options have pluses and minuses.
Option 1 continues is the least confrontational, but doesn’t sound like it will get you to the end point you want. I wouldn’t recommend it as this will not help with your own establishment of yourself as a person. Option 2 is the most confrontational and will likely end in a shouting match at some point. Especially if, as I suspect, you have never gone against your parents in any significant way before. It will come off as something entirely new and combative to them…they won’t understand nor like it. Option 3 is probably the hardest but would likely get you the result you want. But this option will require a good amount of preparation by you.
First, you need to fully understand why you don’t want the training. Is it that the training is in a subject you have no interest in or is it that it will take you away from friends and 6 weeks of doing nothing? Is this just the proverbial straw that broke the camel’s back and you really want to lead your own life? You need to understand your reasons fully before you can articulate them to your parents. I would recommend you have some clue as to what you want to do with your life…have dreams and goals for reaching those dreams. Be able to tell your parents what is in your heart and what you want obtain from life. And when you sit your parents down to talk with them, be the adult in the situation. Be calm and rational. Leave your cell phone somewhere else, turn the TV off…give them your full attention with no distractions. Ask them why they want you to go to this training class (if you don’t already know). Listen to their answers. Explain to them your reasons for not wanting to go. Staying at home for the summer to hang with your friends may be a perfectly acceptable answer if, for example, you are planning on going away to college in the fall and will likely not see them for a long time. Also, going to a training class in a subject you have no interest in is a huge waste of money.
Again…make it a discussion. This means talking but also listening. It means remaining calm. Don’t discount your parents feelings. They want something for you…you need to recognize that as them caring for you. Don’t ball up their feelings and throw them back at them. But if you want to be an adult, be an adult.

See @Dutchess_lll above
Exercise your legitimate 18 year old option to get a job and move out
Unless of course you are dependent on your parents and intend to stay that way. In that case you’ll be going to camp.

Are they going to be away and they don’t want to leave you home alone? Perhaps in the past, they’ve left you home alone and you’ve done something you shouldn’t have done, like have a party? There are two sides to the story. We’re only hearing one.

You’re an adult. They can’t make you do anything.

If you’re from a western country then they’re probably telling you to GTFO, find a skill or job and stand on your own feet. What training is this? If they’re paying personally I’d jump on it. Free training you have time to attend in anything useful jump on it. Opportunities like that are few and far between. If you’re living on your own and really don’t want to go then why even ask, you don’t have to go and your folks are being too controlling.
Are you Asian? I understand that the culture is different and it’s harder to say no but you still can provided you’re able to live on your own and are ok with the consequences of doing so. A little bit of advice though…. your parents know whats best for you 98% of the time….

Also, you need to respect all of the people here who answered your question and follow up with a response.

We are never going to hear back from the OP, are we….

No.

Divorce them.