What will happen if my Apple ID saying Sign out is not available due to restrictions

If an app or feature on your device is not working or is restricted, it may have been disabled using Screen Time, or by an IT administrator if you are using a device owned by an employer or educational institution. See how to use Screen Time on an iPhone, iPad, or iPod touch by clicking here --> Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support

Related information about how to use parental controls on your child's iPhone, iPad, and iPod touch to restrict access to features --> Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

Most of the times these are screen time restrictions.

Go to settings>screen time>content and privacy restrictions and turn restrictions off. And try to sign out again