@Usman Abdullahi, Thanks for posting in Q&A.
From your description, I know you have configured an EDR policy to onboard devices but some of devices not onboard to MDE and show an onboarding status of 'can be onboarded' in MDE
Here is a link you can refer to troubleshoot onboarding issues.
Also, there are some pre-requirements you need to check if you meet and this issue can be related to License, please also check if the necessary licenses were assigned to the device.
https://learn.microsoft.com/en-us/defender-endpoint/minimum-requirements#licensing-requirements
If there is any update, feel free to let me know.
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