Hello Mahadi Hasan Tuhin, welcome to the Microsoft community.
Based on your feedback, you're facing issues with Windows 11 activation and Office constantly crashing.
Regarding the Windows 11 activation issue, I'd like to confirm with you:
Have you activated Windows before?
You can press windows+X and choose "Windows Terminal (Administrator)", then enter the following command:
You can directly copy: (Get-WmiObject -query 'select * from SoftwareLicensingService').OA3xOriginalProductKey
Then check if a key appears, and click on Settings > System > Activation > Enter Key
Then check if it's activated.
The Office issue is handled by another team.
So, I'll first see if I can help you resolve your problem.
Hope the above information is helpful to you.
Kirito | Microsoft Community Support Specialist