Apple ID account issue for Samsung user on Apple one Family Plan

My daughter just recently upgraded her samsung phone. One of our upgrades is the Apple One Family Plan. I invited my daughter to join the plan and when it asked her to sign in to her apple account, it says "This action could not be completed because of an error." She had since restored her old phone to factory settings and is now using her new Samsung phone and she STILL cannot log-in to her apple music using my invite. She called apple support and after trying all her emails and phone number, they say her account does not exist. How has she been using apple music all these years, then? She finally created a NEW apple ID account. i sent her another family plan invitiation link. and she still receives the "This action could not be completed because of an error." HELP!

[Re-Titled by Moderator]