I want to bring in @dmertz because they are the absolute expert on this topic.
If you fail to repay the loan within 60 days, it becomes a code M distribution from Vanguard. That's the end.
However, you are allowed to make a rollover contribution to a new custodian by May 17, 2021 (if the offset distribution for the loan occurred in 2020). This is an extension of the normal 60 day rollover rule. If Ameritrade won't accept a payment as a rollover of an offset loan, contact another investment house and find one that will accept the rollover. Don't ask them to accept a loan repayment (which they can't in any case, because it was not their loan to begin with), ask them to accept a rollover contribution, and any plan should accept a rollover.
Then on your tax return, you report the code M, and turbotax will ask what you did with the money; you say you rolled it over, and all will be well.
Vanguard can't accept a loan repayment because you closed your account. Ameritrade can't accept a loan repayment because it was never their loan. But you aren't making a loan repayment, you are making a rollover. That should be simple.
File form 4868 for an automatic extension so you have enough time to straighten this out.
