Thanks for reaching out. If you can't see the option to manage your family plan members in your account page here, it's possible that you're not logged in with the plan manager's account. I'd recommend you try logging in with other email addresses you may have or try these steps in case you can't find another account.
Let me know how you get on!
Help others find this answer and click "Accept as Solution".
If you appreciate my answer, maybe give me a Like.
Note: I'm not a Spotify employee.