Coursera - submission problem

I have seen that error from other students in the past. It means that you have accidentally (or purposefully) deleted some of the comment lines in the function cells that the grader depends on. You should not need to modify anything except in the “YOUR CODE HERE” sections. It’s not illegal to make other changes, but you need to be careful and know what you are doing.

My suggestion would be to get a clean copy of the notebook using the procedure on the DLS FAQ Thread and then copy just your function code over to the new notebook. Or compare the function cell that has the UNQ_C1 tag and copy the appropriate comment lines to make the first few lines of that cell look like the clean version.