Unable to enroll in MDM

Do you want the users to enroll their devices themselves or do you want e.g. IT to prepare the device and handling enrollment when the devices are set up initially (via OOBE)? 

I also suggest you read up on the topic Entra ID registered / Entra ID joined, and the difference between adding a device in Entra and enrolling the device in Intune. 

If you're using Autopilot enrollment from OOBE, make sure the device is added in the Autopilot devices list and the device has an Autopilot profile assigned. You can achieve this with a dynamic device group and by following these steps: https://learn.microsoft.com/en-us/autopilot/enrollment-autopilot#create-a-windows-autopilot-device-group-using-intune 

If in Intune's enrollment restrictions you're blocking personally owned devices, you can't enroll it simply by adding the work account in Windows settings. You would also have to make sure the device join settings in Entra ID under Devices > Device settings allow the Entra join specific to your environments conditions (if you're requiring MFA for the join for example).

You can also use the event viewer to troubleshoot the auto enrollment process: https://learn.microsoft.com/en-us/windows/client-management/mdm-diagnose-enrollment#troubleshoot-group-policy-enrollment