@riahbearrr A stuck imaginary friend can wreak havoc on a save, whether the IF is already in a sim's inventory or still in the mailbox. If evicting the family and placing them back in their house didn't help, you'll need to get rid of the IF. Here's how:
https://www.nraas.net/community/Which-Mod-FAQ#Im-having-problems-with-the-Imaginary-Friend-Dolls-in-my-town-What-can-I-do
The first option is for one IF at a time; the third is for all the ones in town, and it may or may not be necessary. If you install ErrorTrap and are seeing errors for other IFs, that would strongly suggest it's time to flush them all.
The first option can also work for an IF stuck in a mailbox or in a sim's inventory. In that case, click on the ground of the lot and select NRaas > MC > Object Stats > Inventory, then find the IF and flush it. This only applies to objects on the current lot, including in inventories, so other IFs in town would be unaffected.