Unable to use settings

Do you have a MDM profile?

If so, contact your company - IT department.

or

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.

Mobile Device Management security overview:

Mobile device management security overview - Apple Support (NL)

Contact your company / organization - IT department.

The iPhone belong to a company / organization.