Cannot set parents for pre-existing sims

Does anyone else seem to have this issue where you cannot create parents for currently existing sims? I have an alternative save file that I brought my sim into to create his family, but EVERY TIME I go to make his parents, THE GAME DOES NOT RECOGNIZE THE RELATIONSHIP!!!!!!!!!!! And I can't even set the relationship to parent/son/daughter unless I make brand new sims. There is no way I can make the parents exist for my sim without having to resort to modding. I've used the CAS cheat, I tested with a new household, which it works perfectly then, I aged down my sim to create a larger age gap, but still nothing works. I cannot make the parents for the existing sims I've downloaded. Is this a bug, or glitch, or genuinely does the game not let you play with genetics unless it's for a child? This is an extremely frustrating thing. Why can't the game allow me to create my sim's parents??????? Also why is it never an option in CAS when editing familial relationships? All I ever get is brother, partner, spouse, engaged, or household member options for sims I've downloaded.

Images below show the issue. The sim I downloaded is David Garcia, and I tried to create his dad, then added a random child to show that I could create his daughter, but the parent I attempted to make isn't recognized nor unable to be set as the father for David. Why????