What is the code in box 7 of the Form 1099-R provided by the payer?
Is the IRA/SEP/SIMPLE box marked?
If the 2018 Form 1099-R in your name and SSN or your dad's name and SSN?

TurboTax would be displaying this message only for a code P or code R 2018 Form 1099-R, neither of which should be on a Form 1099-R for a distribution made to you.

Thanks for your response! Box 7 has a P. The IRA/SEP/SIMPLE box is not checked. I don’t have it in front of me, but I think my dad’s name is on the 1099. I have to double check that though.
If my name was on it, would the code in box 7 be something else or would that box be checked?

This seems a bit unusual, but perhaps is correct and needs to be reported on your dad's 2017 (final) tax return.  It seems to be a return of an excess elective deferral your dad made in 2017, perhaps due to the 401(k) plan not passing the nondiscrimination tests since it seems that your dad could not have personally requested this distribution (unless he made the request in late 2017 before he died and the distribution was not completed until 2018).

It seems unlikely that this would be the only money that was in the 401(k) unless his death in 2018 somehow made him ineligible to make elective deferrals to the 401(k) plan for 2017, which seems unlikely.

A Form 1099-R reporting any distribution paid to you as beneficiary should have code 4 in box 7.  I think you need to contact the plan administrator to be sure that the reporting is correct and to make sure that any remaining balance in the 401(k) is dealt with appropriately.

Just looked at the 1099R again. It has my name as the recipient. Box 7 has a 4. Oddly, when I looked at it in turbo tax, it had changed box 7 to a P. The strange thing is I also attempted to do this in HR Block website and got a message saying the same thing turbo tax said. So if I understand you correctly, since the 1099R is in my name, box 7 has a 4 and the IRA/SEP/SIMPLE box isn’t checked, this message shouldn’t be coming up?

Box 7 is a user enter box.   I know of not circumstance where TurboTax would (or could) ever change the box 7 entry.   I suggest deleting the 1099-R and re-enter and be sure to select "4" from the box 7 pull-down menu.

I really appreciate your time spent communicating with me about this!