Is your mother trying to get an FSA ID? If so, then fill out the box as Did Not File, and then follow the instructions. They may require you to get a Non-filer’s statement (she gets that through the IRS) for the FAFSA, but for the FSA ID, she may not need that if they can confirm her identity through her SSN, birth date, address, etc.
Always answer the questions truthfully. Did she file 2019 taxes? No. That’s the answer. It may cause you to have to provide more forms or info, but that’s the answer.