I got errors in test functions that I can't change or delete

That message simply means you have not run the “import” cell early in the notebook which imports those test functions from the file public_tests.py. Try “Cell → Run All Above” and then run that test cell again.

FWIW there is a topic about this on the DLS FAQ Thread, which is worth a look in general if this is news to you.

If you try my proposed solution and it doesn’t work, then it must mean you have accidentally damaged the notebook in such a way that the “import” cell no longer works. E.g. maybe you accidentally turned it from a “code” cell into a “markdown” cell. In that case, the solution is to start with a clean copy of the notebook and carefully copy over just your completed work from the “YOUR CODE HERE” sections. There is also a topic about how to do that on the FAQ Thread.