This message/error will be seen when the .codio file is incorrectly formatted.  Either the student has tried to enter in a new command incorrectly, removed something  or even tried to write code into this file in error

As a teacher. you can view the students project (see Accessing student work) to see what the problem is in this file (the project will open in a new browser tab) so you can then compare it to what you can see if you were to open the same project in the course from your original browser tab where your .codio file should be correct

You should then be able to copy/paste the correct content into the students .codio file so they will then be able to carry on, but also you can let them know what they may have done to cause this so they will be less likely to repeat this in the future