Trying to add new iCloud email onto iPhon, keep saying to Apple ID doesn’t work

Synopsis: I was family sharing my own Apple ID for purchase, music and so on with hubby and still wish to share.

Factory reset my old MacBook Air and gave it to hubby. Created an Apple ID for him to receive iCloud email on it.

When trying to add iCloud email to his phone so he can receive his iCloud email onto it also, it doesn’t recognize his new Apple ID and it either says it’s not valid or already in use.

what am I doing wrong?