question

Hello @PrimaryUser-0033

I would suggest you to verify below
1. Make sure you have enough permission to modify objects in intune - preferably Intune admin role
2. Only devices that are hybrid joined or azure ad joined are allowed to have primary user field modified. If the device is AAD registered, it's not allowed
3. Primary user are populated during auto enrollment process, so ensure the user in question has auto enrollment policy applied.
>>INTUNE>Devices>enroll devices > auto enrollment> should be set to specific group and user has to be part of it or to all users

Regards
Aravinth M
Please mark as answer if tt was helpful

@PrimaryUser-0033 Thanks for posting in our Q&A.

Based on my understanding, the primary user can distinguish if the device is a "Shared Device". When there's no primary user assigned, the device is referred to as a "Shared Device".
Intune automatically adds primary user to devices during or soon after enrollment.
https://docs.microsoft.com/en-us/mem/intune/remote-actions/find-primary-user#who-is-assigned-as-the-primary-user

"Enrolled by" is the user account we used to enroll the device to intune.

As AravinthMathan said, we can't change the primary user if the device is AAD registered. For windows devices, if the device is Azure AD join (bulk enrollment token) or Azure AD join (Autopilot self-deploying mode), it doesn't have the Primary user.

If possible, please tell us what the enrollment method did you use to enroll the windows device. The enrollment method is in the following article:
https://docs.microsoft.com/en-us/mem/intune/enrollment/device-enrollment#windows-enrollment-methods

If there is anything update, feel free to let is know.

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