Apple id

Do you have a MDM profile?

If so, contact your company - IT department.

or

If it is a private iPhone, see -> Use parental controls on your child's iPhone, iPad and iPod touch

Use parental controls on your child's iPhone, iPad and iPod touch – Apple Support (UK)

• Check restrictions -> tap Settings -> Screen Time -> Content & Privacy -> scroll to Allow Changes.

Try this -> Check restrictions -> tap Settings -> Screen Time -> Content & Privacy -> scroll to Allow Changes.