1099-B box 1b is blank which is an option and listed in help text as valid choice but TT review keeps flagging blank as an invalid choice

[HelenC12]

For financial institution reporting to you, yes, blank is a valid  choice. Your financial institution doesn't have the date you purchased the stock. However, you, as the taxpayer, needs to enter a date that you acquired the stock. The IRS needs a date so they can determine how they're going to tax you.  A short term stock sale is taxed differently than a long term stock sale. 

Try to narrow the purchase date down to a date or a date range. Document how you came up with a date. Once you have that, follow these instructions to locate historical prices.