Your question has been answered quite well already, but I understand your fear of doing something wrong with your first job. I know at this point the more support you can get for how you handled things, the less you feel that knot in your gut.
You did just fine.
Here is the scene…
Jobs which involve lots of teens and/or single parents (sick kids) on the schedule tend to have employees calling in at the last hour, or just not showing up.
As a result, they are going through their list of people who might be able to fill in. Doing this is a nightmare, and they have to just hope they can find someone with basically no life. They actually PREFER a quick yes or no, so they can fix the problem quick as possible.
What you do with your time is none of their business, and mostly they couldn’t care less anyway.
If you want to help out, but you really do have previous plans, you might say, I have plans, but, if you try everyone else and don’t get someone call me back, K?
The better you can help, the more they come to appreciate you. The quicker they move on when you can’t help is also appreciated.
It is not like school. Your boss doesn’t want a story. They just want the shift to go smoothly.
Now, if you are scheduled, and you want out of it, some story might be expected.
Since your boss asked in a text, they probably sent one text on a list type send, to hit everyone at once, and waited for people to respond. Gawd, sometimes technology is so great. Imagine, in my teen days, to reach all employees like that required separate calls (no texts back then), punching seven buttons, waiting through rings for someone to answer, usually a sibling or mom, waiting for employee to get to the phone…
Well, a list text must be a sweet change for managers!
You see how they just want a quick, no frills answer?
Again, you did just fine.