My ex-husband started our tax return but has not submitted it how can I get into that and get it submitted

You probably cannot unless you have the account ID and password and can get the security verification code via phone text or email.

Unless you were divorced in 2020 and it was not final in 2019 the  there can not be any "our" return.   If not married at the end of 2019 then you can only file as single or Head of Household of you qualify.