sims 4 won't let me move a sim in with my sim

If you're asking to move another sim in your current household which isn't created yet, you have to go to Manage Worlds which is automatically there when you press the Play button. Click onto the household's home, press the "..." Icon for More options for the Household. Then, click onto Manage Household and then press Edit, Add, or Remove Sims from the Household." There you can edit your household Sims, Add or Remove them.

If you're trying to move in a sim already created, save the household of the Sim you want to move into the new household, you have to merge the new household with the old household.