@Chris Yue - I was having the same issue. The device was Azure AD joined and I when I checked my InTune permissions, it showed, "You are an administrator with full permissions to all Intune resources." I put in a ticket in with MS and we found the issue.
The main issue was the MDM User Scope found under Devices | enroll devices | auto enrollment - it was set to none and needed to be set to all. After this, the primary user could be changed with any newly enrolled device. Also, company portal is only really needed if you want your users to see software available to them without installing it - it is not required to join the device.