How to available Bluetooth in iPhone

If your Bluetooth setting is greyed out, try to force restart the phone and make sure that no restrictions are set up in ScreenTime.

Force restart iPhone - Apple Support

If this does not work, and to rule out a software issue, set your phone back to factory settings, using your computer.

Then set it up manually, without using the backup see if you can enable Bluetooth.

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

If I remember correctly, some iPhone models had a problem with the Bluetooth chip, that has not been welded correctly onto the motherboard and lost contact over time, resulting in a loss of Bluetooth.

You can enable Bluetooth on iPhone by swiping down from the top right of your iPhone screen and tapping the icon next to Wi-Fi.

Alternatively, open Settings > Bluetooth > Bluetooth On.