I have just given my iPhone 8+ to my sister, having performed a reset first. She wants to change the Apple ID on it to her own but, whenever she tries, it comes up with an error message:
"Email used by another Apple ID. This email address is not available. Choose another address."
Having googled this error message, it looks like the problem might be because, in the past, she changed the email address associated with her Apple ID. In which case, in order to solve the problem she needs to login to id.apple.com using the old email address/Apple ID, and then remove the new one from the list of alternate emails.
Unfortunately she can't login with the old ID because she doesn't remember the password or security questions. Any suggestions? Also, could there be any other reason for this error message coming up?