Unable to pair Apple Watch that shows sister's Apple ID

Lent my watch to sis for a week and now her phone is saying it’s no longer paired (not showing up in all watches) but my watch shows her Apple ID and won’t pair with mine again.

I tried sending a verification code to her phone from the watch “password and security,” hoping it would let me then change it, but when I’m in her phone, nothing is popping up for me to put the verification code into. Watch says “enter this verification code on your other device to sign in,” followed by numbers but no details where I’m supposed to enter it and her phone isn’t getting prompted. Everything is connected to Internet.

Of course my sister’s repaired watch immediately paired to her phone.

[Edited by Moderator]