I do my parents taxes (married filing jointly) each year with TT. My dad passed away in 2020 and I too was also getting the same error for AGI or Pin incorrect and getting rejected. My dad was always listed first when using the software. I flipped it around so my mom was listed first in the personal info area, making sure to check that my dad died and the correct date of death. Also make sure birthdate for each person is correct. Along with that i had to change the 1099R and SSA information since the values had to match the correct person. When submitting, I entered the AGI from the previous year, entered my mom's 5 digit PIN, submitted and was accepted!!!! WOOHOO!!!
Another reply from TT says order doesn't matter, but for me it did. So put surviving person first, person who died second in the personal info area. Be sure to check wages/income to match the correct person as that doesn't seem to follow/update after you update the info section. Use same AGI as previous yr. Good luck!