My company went bankrupt and the pension plan has been administered for the last 20+ years by the PBGC. My 1009R shows 0$ in box 2A for the taxable amount. What to do?

What type of pension is it?  If this was a standard pension, then the taxable amount would not be $0. Pensions are taxable by the IRS. TurboTax is asking how you figured this, because it may be wrong so it is asking how you got the number.  Is this the first year you received the pension?  If not, what was the amount in box 2a last year?  Is there a reason it should NOT be taxed?

If it is wrong, it will change the amount of taxes you owe, then if ever caught, there could be fines and penalties for underpayment.  Unless you KNOW there is a reason that it is not a taxable pension, then you should take the number out of box 2a and leave it blank. 

If it is correct and there is a reason your pension is not taxable, then leave it at $0.