How to resolve 'Already part of a family' error in Family Sharing?

I am getting confused as to exactly which accounts are being used and where. While you generally sign into Media & Purchases as part of signing into everything on a device, you can, in fact, be signed into a device itself with one Apple Account but be signed into Media & Purchases with a different one. Now add onto the fact that you were talking about sharing an account with your partner and then talking about having your own account.

The bottom line is, an account can only be part of one family group at any one time. If your partner has a Family Sharing group set up, your partner cannot then join one you have set up. You will have to decide who will be the Organizer and then the other person should disband their group (How to leave or remove a member from a Family Sharing group - Apple Support). Either of you can take out the Apple Music subscription and share it with others in the family group, but it will have to be a family level plan. More about that here --> Add a family member to your shared subscriptions - Apple Support