Did you start the return as a joint/coupled return? Because only Married Spouses & Common Law Partners can prepare their returns together.
To be able to choose any other marital status, you'll have to prepare a single return. When you start a new return, on the Welcome to TurboTax page, you would answer that you are preparing the return for "Someone Else" and then that you are NOT preparing a married or common-law return.
If your mother only became a widow in 2019, then her deceased spouse/partner’s return would have to be prepared separately. This FAQ has more information regarding preparing returns for the deceased: https://turbotax.community.intuit.ca/community/product-information/help/can-i-prepare-a-return-with-turbotax-for-someone-who-has-died/00/873466