im unable to access my icloud on iphone. how do i reactivate it

See -> If your Apple ID is greyed out in Settings

If your Apple ID is greyed out in Settings - Apple Support

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If you can't click your Apple ID

1. Go to Settings, then tap Screen Time.
2. Tap Content & Privacy Restrictions. 
3. If Content & Privacy Restrictions is turned on, scroll down to the Allow Changes section.
4. Tap Account Changes, and set it to Allow. If asked, enter your Screen Time passcode. 

If you can't click other options in Settings 

1. Go to Settings, then tap Screen Time.
2. Tap Content & Privacy Restrictions.
3. If Content & Privacy Restrictions is turned on, scroll down and check if any settings are set to Don't Allow. 
4. If so, tap the setting, then choose Allow. If asked, enter your Screen Time passcode. 

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.

or

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