I tried everything and nothing worked, then one day I decided to call IRCC and spent 2 hours in line waiting. They solved the issue. It was simply that the number of applicants I was entering was wrong. At that point in the link application menu, I was always entering 2 applicants because it was for my mom and dad, however the number they told me over the phone was 4. The logic was that there were my 2 parents and then myself and my co-signer. It's nonsense since I also have kids as dependents, but that was the issue. so I recommend you try different numbers of applicants.