Please make sure the account that login to Microsoft Endpoint Manager admin center is either an Intune service administrator, or a tenant administrator with the global administration permission, meanwhile, the account should have been assigned an Intune license. Also, A user must have an Intune license to be assigned as a Primary user and updates to the primary user across Endpoint Manager and and Azure AD can take up to 10 minutes to be reflected.
In addition to the Microsoft Endpoint Manager console, you can also try to change the Primary User through graph API. Here's the Powershell script from Github: https://github.com/microsoftgraph/powershell-intune-samples/tree/master/ManagedDevices#1-invoke_devi....
Reference: https://techcommunity.microsoft.com/t5/intune-customer-success/change-the-intune-primary-user-public-preview-now-available/ba-p/1221264
https://learn.microsoft.com/en-us/mem/intune/remote-actions/find-primary-user
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