If CSF 1099r box 2a in unknown, what do i put down

No. you can't leave it blank. After you enter the 1099R, there will be a series of questions to help determine the taxable amount of the distribution.

1. After entering and as you progress through the interview, there will be a screen that says, 'let's figure out how much of this is Taxable. Then check the check box stating you need to figure out how much of this money is taxable. See screenshot 1.
2. Then a screen will appear asking what date the annuity started, the plan cost ( this should be listed in Box 5 on the 1099R), number of months you received payments in 2023, death benefit exclusion. ( leave this blank unless the deceased passed away before 1996). See screenshot 2
3. Next screen asks if this covers you or is it a joint annuity.
4. Next it asks about retirement age and if you are under retirement age. Retirement age is usually 55 for these types of plans.
5. Finish out the section.

Once done, the taxable amount will appear in Box 5B of your 1040.