You should first be certain that the entire investment has become worthless or that no more funds are going to be sent to you to make you whole. If there is still a possibility that more settlement funds will be forthcoming then you need to wait to enter the transaction.
If you have sustained a loss due to a bankruptcy of one of your investments then you should enter that complete loss manually into the TurboTax system.
Enter it as a stock transaction without a 1099-B. You enter the date of purchase as the date that you made the initial investment and the purchase price as the amount that you had invested. Where do I enter investment sales?
Cryptocurrency Guide
Community - Detailed Example