If a Windows 10/11 device was already joined to Azure AD or Entra ID before the enablement of the device automatic enrollment, then the device has to be manually enrolled by one of the following ways:

Manual Enrollment via Settings

This is a straightforward option for smaller deployments or scenarios where IT can configure devices individually.

Steps:

1. On the PC, go to Settings > Accounts > Access work or school > Connect.
2. Select Enroll only in device management.
3. Enter the work or school account credentials.
4. Follow the prompts to complete the enrollment process.

Install the Intune Company Portal app 

https://learn.microsoft.com/en-us/mem/intune/user-help/sign-in-to-the-company-portal