Can I file an extension for a past year? I'm filing an amendment for 2021 and if I had filed an extension for that year, it would help me with my amendment.

Huh?  You have 3 years to amend a return.  Did you file the original return?  Why are you amending?

No.   You have to request an extension by the April filing deadline for the tax year.  After that, if you do not file on time, your return is just late.  If you do not owe tax due, then no big deal.  You have three years to file a tax return if you do not owe or are getting a refund.  You cannot reverse time and request an extension for your 2021 tax return.