I have created a separate user for my GF on my mac (macos Catalina 10.15.4) but I cannot log into FaceTime and iMessage. FaceTime shows "this account has no reachable addresses" error, but account is connected to appleid and works on her iphone. iMessage just shows incorrect login or password. I tried re-login with my credits and it works fine. My GF has such troubles with FT and IMsg on her phone about year ago, but after updating on 12 ios this problem disappeared.
It seems like something wrong with her AppleID, her Icloud profile can't load her avatar on MacOS and sometimes there are troubles with updating info, it doesn't save data.