unable to remove parental controls

I cannot register it, because it says the serial number is invalid

it says create a family group

You can troubleshoot here: https://beta.support.xbox.com/help/family-online-safety/browse

Was the account made under a family setting? may be locked due to the family settings on other account. If theres a family account, you would have to access that account to remove an account connected to it.

Hi There,

Open a web browser and type http://account.microsoft.com/family in the address bar. In the webpage that opens, sign in with your account. You will
now have two sections, adults and children. Select the one you want to remove from privacy. If you want to remove yourself, you will be in the adult section. However, you will have to remove all the children before you remove yourself.
Go in either of the section, select the account you want to remove from privacy and click Remove.

Click on the heading that says User Accounts and Family Safety. Once you are in it, you will see Parental Controls. Click on it and set it to off. This will turn off the parental
controls that might be on for any user account. Keep in mind that the parental account will have to login and change all of the settings for the child account.