probsoln package FAQ
You need to define a problem before you can use it. Check to make sure you haven’t mis-spelt it, and check to make sure you have used the file in which it is defined either via
\loadselectedproblems. If you have used
\loadrandomproblems, you shouldn’t try referencing a problem since there’s a chance it may not have been selected.
Alternative link: https://www.dickimaw-books.com/faq.php?itemlabel=prslabelundef