Form 4852 is a substitute Form 1099-R. I can't imagine that you would need to do a substitute Form 1099-R for a distribution from the federal TSP. It would be unusual for the TSP to issue an incorrect From 1099-R or fail to issue a Form 1099-R for a distribution.
Somehow you got a Form 4852 linked to your Form 1099-R for a distribution from the TSP and did not provide a description of your efforts to obtain a correct Form 1099-R from the TSP. If you really have a reason to file a substitute Form 1099-R for a TSP distribution, you'll need to provide the missing explanation. If you have no reason to file a substitute Form 1099-R, delete and reenter the Form 1099-R for the TSP distribution.
I spent a month trying to get my TSP 1099R to enter correctly. My wife and I had identical situations. We both received part of our RMD's for 2020. When CARES ACT was passed and we found we could rollover part of those withdrawals. Our 1099R's did not reflect this rollover. We also had a basis and should have 8606's generated by Turbo Tax. Turbo tax would not generate a 8606 for me as taxpayer but would generate one for my spouse. After trying every suggestion from the forums and calling Turbo-Tax I still could not fix this problem and was about to send in a paper return with 4852 because I was forced to override entries on IRA tax summary. The 1099R from TSP did not have IRA/SEP/SIMPLE block checked which was strange. So before sending in the return, I experimented with checking IRA/SEP/SIMPLE block on the entry form for the 1099R and everything worked! Turbo tax rep did not suggest this when I called, but a simple fix and accurate as far as I could tell.