Check each imported transaction. If any show a date a various then enter the correct date. If you continue to receive the error then delete the 1099B and enter the information manually. If you have a lot of transactions then it would be best to enter a summary for your transactions instead of individual.
ttlc.intuit.com/community/entering-importing/help/how-do-i-enter-a-large-number-of-stock-transactions-in-turbotax-online/00/26748
