I cannot select in setting menu, its display ed in lighter color

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.

or

If you have a MDM profile, you need to contact your company - IT department.

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