Hey Oosikhead,
Thanks for using Apple Support Communities. It sounds like you've taken the correct steps to add the Downloaded music option to your Music app. We'd recommend checking to see if any restrictions are enabled next. If there are, that might be why you are unable to make these changes. Check out Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support which has more information regarding restrictions. We've included a couple sections below that might be particularly helpful.
"Set Content & Privacy Restrictions
- Go to Settings and tap Screen Time.
- Tap Turn on Screen Time, then tap it again. Choose "This is My [Device]" or "This is My Child's [Device]."
- If you're the parent or guardian of your device and want to prevent another family member from changing your settings, tap Use Screen Time Passcode to create a passcode, then re-enter the passcode to confirm. After you confirm your passcode you'll be asked to enter your Apple ID and password. This can be used to reset your Screen Time passcode if you forget it.
- If you're setting up Screen Time on your child's device, follow the prompts until you get to Screen Time Passcode and enter a passcode. Re-enter the passcode to confirm. After you confirm your passcode you'll be asked to enter your Apple ID and password. This can be used to reset your Screen Time passcode if you forget it.
3. Tap Content & Privacy Restrictions. If asked, enter your passcode, then turn on Content & Privacy Restrictions."
"Prevent explicit content and content ratings
You can also prevent the playback of music with explicit content and movies or TV shows with specific ratings. Apps also have ratings that can be configured using content restrictions.
To restrict explicit content and content ratings:
- Go to Settings and tap Screen Time.
- Tap Content & Privacy Restrictions, then tap Content Restrictions.
- Choose the settings that you want for each feature or setting under Allowed Store Content."
All the best.