I had this problem on two Windows 11 PCs. Simply turning Find My Device off and back on did not fix it.
What worked on both machines was deleting the stale ChannelUrlExpiration registry value and then toggling Find My Device.
Open Command Prompt or PowerShell as Administrator and run:
reg delete "HKLM\SOFTWARE\Microsoft\MdmCommon\SettingValues" /v ChannelUrlExpiration /f
Then go to Settings → Privacy & security → Find my device, turn Find My Device Off, then turn it On again.
Windows will recreate the registry value automatically. Find My Device then worked immediately on both PCs.
If the usual Off/On fix isn't working, this may be worth trying.
As always when modifying the registry, make a backup first.