Hello sdtd,
Welcome to Apple Support Communities!
We understand that while listening to music on your device, you’re frequently seeing a message prompting you to change your subscription to a family plan and despite selecting “not now”, the messages keeps showing up. We’d like to help.
We see that you've already checked out Change your subscription from Apple. Did you recently make any changes to or cancel your Apple Music subscription?
Try restarting your iPhone and then reopen the Music app. Does the issue continue after closing the app and restarting your device?
Is the iOS on your device updated to the latest available version? If not, back up and Update your iPhone, iPad, or iPod touch.
If you continue to see this prompt after trying those suggestions, contact Apple Support. Here’s how to Get Support.
Have a great day.