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 methods:

Manual Enrollment via Settings

This is a straightforward option for smaller deployments or scenarios where users or IT administrators can enroll 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 

Intune Company Portal is the app that lets you, as an employee or student in your organization, securely access your apps, data and resources.

Please refer to the official guide below.

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