Merging housholds impossible

@peno11 There's currently two issues. Could you describe in more details what you are seeing ? Maybe add some pics ?

@peno11 

Households in The Sims 4 can only have 8 sims.  Included in that count are the C&D pets.  This is hard coded into CAS.  So, you have one more sim between the 2 households than the game will allow in a single household.  So, in your case, the only way to combine the households is to either move or delete a people sim or a cat to get you down to 8 sims.  

If you play with mods, there is at least one mod that I know of that will let you have more than 8 sims in a household in live mode but even that won't let you merge the two households. The household can grow to more than 8 sims, but you can't merge other sims into the household to go above 8.  And using that mod option is somewhat risky because it requires you to edit one sim at a time in CAS using a separate option in the mod.  If you forget and take the entire household to regular CAS where the entire household goes, a random household sim will get permanently deleted as the 8 sim limit is enforced by CAS.  There is no mod that I know of that will let you take more than 8 sims to CAS.  Because of that drawback, I don't recommend having households above 8.

Hope this helps.