I've tried entering values that haven't been used or setting to NULL, but still getting that error message, preventing me from importing.

Probably you have lines after your data that may be cleared of data, but still have what I call "residue" that the import tool can see, even if it's not visible in Excel. So if your spreadsheet you started with initially had 100 lines, but you're only importing 50, the import tool may "see" data on lines 51-100, even though there's really not. So since it thinks you're trying to import lines 51-100, it's bombing out because mandatory fields aren't filled in for those lines. Pretty easy remedy. Before I save an import file I select the first blank line after my data and delete all lines from there to the end of the spreadsheet. I do the same thing starting with the first column to the right of my data. That typically will take care of it.