Since you haven't yet filed, you might try going back into your return and change filing status from separate to joint and make any other required changes. I don't know if TT will let you do this but you might try it. If not, then the easiest thing to do would be to go ahead and file and let your husband also file separately. You might also contact TT (see Contact Us link under Help and Support at bottom left of page) and see if they would credit what you have paid to a new account that you would file jointly. But good luck with this.
If you want to change your return to Joint but it doesn't let you back in, Click on Add a State to let you back into your return.
Or will you be using a different account like his to do the Joint return? Did you get married in 2020? Or how did you file last year?
If your spouse has already e-filed their separate return, the only way to fix this issue is for your spouse to file an amended return that changes their status to married filing jointly and adds your information and income. This must be done before April 15, but it cannot be done until after the IRS has processed your tax return and issued whatever original refund was claimed. Meanwhile, you simply abandon your unfiled a tax return and do not file it. You would have to contact customer support about a refund.
If neither you nor your spouse has transmitted any tax return, then it is a simple matter to go back to the beginning and add your spouse‘s personal information and change the filing status. The only thing you can’t do after paying is “clear and start over” to get a completely fresh and blank slate. If you want to clear and start over, you must contact customer service. However, there should be no need to do that. Just go back to the personal interview, change your status and add your spouse, then review all the sections of the program to make sure that you add your spouse’s income, deductions, and credits.