Purchased a used turbotax cd. I finished my taxes but can't efile because it's already registered on another computer. How can I efile?

I'm not sure who you purchased a "used" CD from, but you got scammed.

TurboTax desktop software is sold to be utilized by one user on that user's computer(s) only.  It is a violation of the end-user agreement to sell or purchase software that has already been registered on another computer.

You won't be able to e-file your return using this software.

then how come there are hundreds  of them for sale used on Ebay and Facebook marketplace.

I bought used until 2019 tax year...

... then for my 2020 taxes  I bought one on facebook...

I got half way thru my taxes and it asked for the code...then locked me out saying registered to someone else.