Interesting, I'm seeing the same behavior that you are.  Dec 2020 is not valid but Mar or Nov are fine.  <shrug>  I don't think it matters as long as it's in the 2020 tax year.  I went back to the IRS announcement and there's no indication of what month to select, just the year.  Maybe wait a few more months and see if additional guidance is issued.  Have letters been sent out to folks who deferred?  Maybe there will be additional information there. <shrug>

For reference, this is where I looked:

https://www.irs.gov/newsroom/how-self-employed-individuals-and-household-employers-repay-deferred-so...

You might try calling EFTPS Customer Service at 1-800 555-8778.