Location Services is greyed out

A dimmed row in Settings is a policy, not a fault. Work out which policy first: the fix differs for each, and some can only be lifted by whoever set them.

iPhone and iPad: Screen Time

Apple's article on grayed-out Settings rows names the cause — "you might have certain Screen Time restrictions enabled for your device" — and its own fix runs: Settings, Screen Time, Content & Privacy Restrictions, Allow Changes to Settings, then set the item to Allow, entering the Screen Time passcode if asked. Then check nothing else there is still on Don't Allow.

On a parent's device the same controls read: "Tap Content & Privacy Restrictions. If asked, enter your Screen Time passcode." "Scroll down and select Privacy Restrictions." "Choose a category, like Accounts or Calendar." "Choose Allow or Don't Allow." Apple describes these as covering "preventing changes to privacy settings like Share My Location", but it does not print the full list of categories, so it never names Location Services itself.

A supervised or managed device

"Supervision gives schools and businesses greater control over the devices that they own," Apple writes. To see what is installed: Settings, General, VPN & Device Management, then tap the profile. Apple's management reference lists three location-related restrictions an administrator can set — Allow Find My Device, Allow Find My Friends, and Modify Find My settings ("Users can't change any settings in the Find My app"). It carries no restriction named Location Services.

Supervision is not the same as being watched. Apple states: "The only time your administrator can view the location of your Apple device is if they put your device into Managed Lost Mode," and "Your organization can't track the location of the device without locking it and showing a notification."

Android

Google's wording is the same in shape: "Depending on policies set by your administrator, you might not be able to perform some actions or change certain settings on your device," and the restrictions it lists include using location services. A work profile has its own switch at Settings, Location, Location for work profile — but "If your device is company-owned, you'll receive a notification if your organization requires location access to be enabled for a work app. If location access is required, it can't be disabled." Google notes the switches stack: device location off takes the work profile's with it.

On a supervised Google Account, Family Link can set a permission so that only a parent can grant it: Controls, Signed-in devices, the device, App permissions, the permission, then "Only parents" or "Your child and parents". Google does not name location there, or say what the locked row looks like.