Do you have the information about your husband's income and deductions prior to his death, which has to be included on your joint tax return? Who is the executor of his estate? You need the executor's permission to file a joint return. If you've been separated for years, he may have appointed someone other than you as his executor.
If you have access to his financial records, and the cooperation of the executor, you should be able to find his copy of his 2019 tax return, and get the AGI from there. If you don't have access to his records, you should probably be filing separately. TurboTax is assuming that you have all the information and authorization to file jointly. If you have all the other information you need to file jointly, but can't find his 2019 AGI, you will have to file by mail.
I'm sure there's a procedure for the executor to get his 2019 tax information from the IRS, but I don't know what the procedure is. A tax professional could probably help.
