See the following TSP web page regarding RMDs:
https://www.tsp.gov/PlanParticipation/BeneficiaryParticipants/rmdbp.html
The TSP was correct to treat part of your distribution from the Roth TSP as the RMD for your TSP as a whole and was not eligible for rollover to a Roth IRA because the tax code dictates that the first amounts distributed are RMD until your RMD has been satisfied. By rolling over that distribution you have made an excess contribution to your Roth IRA to the extent that it exceeds the amount that you are eligible to contribute as a new contribution.
TurboTax will only allow a rollover from a designated Roth account by someone age 70½ or over to be indicated in forms mode of the CD/download version. Doing so in this case would be in violation of the tax code as mentioned above, although the IRS has little ability to detect this error without an audit. However, if they do, you still have the excess contribution to the Roth IRA to consider on which the IRS could assess excess contribution penalties on which the statute of limitations clock will not have started to run.
To avoid this outcome, your RMDs for the traditional and Roth accounts needed to be distributed from the traditional account before doing any rollovers. (Actually, the rollovers should have been done in 2018 or earlier to avoid any RMD on the Roth balance.)
What you should have done to correct this situation after the fact is to roll over to a Roth IRA as a taxable rollover a portion of your distribution from the traditional TSP account equal to the amount of your RMD. The taxable result would be the same as if you took the RMD distribution from the traditional TSP account before doing anything else.You would then only have had the excess contribution to the Roth IRA to deal with if you rolled over the code 7B distribution. If you received the distribution from the traditional TSP account in late December 2019, you still have time to do this.