For the IRS required fields a blank is not acceptable. There is a selection that allows you to use 'Various" and it is acceptable.
When you walk through the steps, as you can see from the image below, you must select the check beside 'Something other than a date'.
If you still have difficulty, make sure the date acquired reflects the correct holding period. It's not important other than to have the correct holding period for the results to be calculated accurately for tax on your return.
- Long term is a holding period of more than one year and receives capital gain tax treatment (0%, 10%, 15%, 20% depending on your regular rate of tax)
- Short term is a holding period of one year or less and receives ordinary gain tax treatment (your regular rate)