If you were legally married as of 12/31/18, you would file as married filing separately or married filing jointly (if you both agreed at the time). His SSN is obviously included in a joint return, but is also listed on a separate return. Because his SSN is now locked you will need to file by mail.
If you were legally divorced (unmarried) as of 12/31/18, then you should be filing single or head of household. The SSN block on your ex-spouse should not stop you from e-filing, there may be some other kind of error. If you are using a Turbotax account that belonged to your spouse and are trying to file single in your own name, that won't work, his identity is still tied to his old account even if you try and change the name. You need to make a new account and start from scratch. If you accidentally tried to file as married, you also need to make a new account and start from scratch as single.
