In my rental property info section, I've entered in values in various expenses/assets (depreciation) but TurboTax says "not started" and returns 0 on the summary page.

I don't have any suggestions for you, but wanted to add that I have the exact same issue. I've called in and spent ~90 minutes on the phone with TurboTax support, but no one was able to help me.

I thought it might have to do with some passive income loss limits, but if that's the case, then the interface is extremely confusing. 

I have been on the phone for 2 hours now and no one can answer this question.  It makes me question the validity of the entire software.  Even if I was at my limit due to passive losses it should save it until next year and not show "not started".   I used to love Turbo Tax, but now it is the most frustrating experience.

I had the same problem.  I think I figured it out.  As my ADU was not rented for part of 2020 due to Covid when asked how many days it was occupied and how many days personal I subtracted the occupied days from the total number of days in the year and put in that amount under personal days.   I was not able to add any expenses as other people have reported.  However when I ran the federal review it came up that I should have put zero for personal days.  When I did that all the expenses I had added showed up.

Hwall has the answer that helped me. When I went back and clicked on the ‘?’ by personal days in the definition it says not days you used the property yourself as your primary residence after converting it from a rental to personal property. This turbo tax section is worded very poorly. Apparently personal days doesn’t mean days you personally lived in the home as a primary residence. I moved into a property I had rented to a builder as a model home. I put the days I rented it for business and left personal days blank and it worked. Good luck