Xbox Privacy settings and Multiplayer games not working.

What do you mean the "privacy tab on the Microsoft website has been down"?

I have just signed in on
https://account.xbox.com/settings and was able to adjust my privacy and online settings.

Are you getting a page explaining how to add a child account to a parent account? If so, then a parent has to make the changes for your account.

Ask a parent with adult Microsoft Account to go to
https://account.microsoft.com/family and add your child account to the family

Then your parents have to go to https://account.xbox.com/settings choose your account and make the settings for you.

It may be privacy settings go to settings - account - privacy and online safety - xbox live privacy - custom - view details & customise - communication & multiplayer - you can join multiplayer games - allow

Which game is it and have you got your privacy settings set to allow for online multiplayer games?

Go to settings,, account, privacy & online safety, Xbox live privacy, view details & customise, communication & multiplayer and ensure you are set to access friends game request.

If you are using a child account, you will need an adult in your "family" to alter any settings.

Not sure which problem it is talking about but this LINK will show you how to change privacy
settings on your console. It's most possibly the joining multiplayer games, in the communication & multiplayer area of the privacy settings.

Try going into settings > account > privacy and online safety > xbox live privacy > view details and customise > communication and multiplayer and set you can communicate outside xbox live with voice and text to everybody. hopefully that works!