My CSA 1099R box 2a says Unknown, how do I calculate what number goes there?

Report the amount in box 1 in box 2a.  If you retired on disability and are under minimum retirement age, the disability annuity you receive from the CSRS or FERS is taxable as wages.

IRS Publication 721, page 19, states:

Disability Annuity 

If you retired on disability, the disability annuity you receive from the CSRS or FERS is taxable as wages until you reach minimum retirement age, as explained in this section. However, beginning on the day after you reach minimum retirement age, your payments are treated as a retirement annuity and you can begin to recover the cost of your annuity under the rules discussed earlier.

In TurboTax Online, follow these steps:

• Down the left side of the screen, click Federal.
• Down the left side of the screen, click Wages & Income.
• Click on the drop down arrow to the right of Retirement Plans and Social Security.
• Click to the right of IRA 401(k) Pension Plan Withdrawal (1099-R).
• At the screen Who gave you a 1099-R, select  Office of Personnel Management (CSA 1099-R).
• At the screen Any Periodic Payments? select  Yes, got regular payments from this retirement account.
• At the screen Payment Start Year select Yes started getting payments in 2024.
• At the screen Let’s figure out how much is taxable, select I made no after-tax contributions.