Validation error

Can someone help? I run all the tests in the notebook and it passed, but when I click on validate, it gives this error message (there was an error running the validate command).

I have submitted this work to another class and was able to validate it, but when I select the correct class to submit it to, I keep getting the same error.

Thank you (I have restarted the kernel and have closed and opened Jupiter, but I still have the same error)

Thank you.