@flo – sorry, I guess “warning” would be more accurate here.
But “error message” would generally be used for anything that the computer spits back at you with a purpose other than “everything is great, I did what you asked”.
So it might be “I wasn’t able to do what you asked, and here’s why” or “I tried to do what you asked, and I’m not sure why it didn’t work” or “Are you sure you want to do that, because it might cause other problems?”.
In real use the messages tend to be much more cryptic and sometimes include an “error code” or number to try and narrow things down. Computer error messages are notoriously difficult and vaguely worded.
It could be presented as a popup that you have to click to close (which is common with Windows) or just a line in whatever dialog you’re using for typed commands.
This whole description may be too general. In this case I was just referring to the line in @Symbeline‘s question
a box appears saying that modifying this may make it unusable
The box would be the error message. She paraphrased what it says here, the other details provided might provide some clues as to what’s going wrong if she doesn’t already have it figured out.
Cool?