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.