I did a Roth conversion of an IRA in 2017, which was a rollover from a 401k in 2015. It's asking me to explain excess withdrawal, but I don't believe there was any excess

For a distribution from the traditional IRA that you converted to Roth, the Form 1099-R must *not* have a zero in box 2a.  Regular distributions for a distribution from a traditional IRA, including those distributions that are converted to Roth, must have box 2a equal to box 1, box 2b Taxable amount not determined marked, a code 1, 2 or 7 in box 7 and have the IRA/SEP/SIMPLE box marked.

Thanks dmertz! When I imported my 1099-R, it had box 2a blank. I adjusted it to match box 1 and now it seems to be functioning properly (adjusted my taxes due, and didn't ask about excess contributions). Thanks again for the help!

Answered in the comments.