Husband died in 2020, Fed return married filing jointly, State (PA) return cannot be filed jointly. Husband's return filed, where is mine?

No, you can file jointly. If you have already filed separately from your husband on only your Pennsylvania state return, then your best option would be to amend your Pennsylvania return and file jointly. Otherwise, you will have to create a second login, create a mock-up joint federal return, and prepare and file your state return. 

To amend your return, please follow the instructions on the page How do I amend my 2020 return?. 

Then, as reposted by @ErnieS0, in order to file a joint Pennsylvania tax return as a surviving spouse, you have to put yourself first on the tax return and your deceased spouse second.

To do so, please go back to Personal Info, edit your spouse and replace their information with yours. Then edit your info and replace it with theirs. Please note that you may have to change the ownership of any W-2s, 1099s and other forms in your return so they match the new order. Once you switch the order in personal info everything else will be reversed.

Can a PA resident widow file jointly for year spouse died?