Is your JAMF hosted as onPrem or is it hosted as JAMF Cloud, DDM is available only on JAMF Cloud

Jamf Pro automatically enables declarative device management on devices that meet the following requirements:

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  Computers with macOS 13 or later

  
  


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  Devices with iOS 16 or iPadOS 16 or later
  
  

We are on Jamf Cloud and all requirements are fulfilled (OS Version etc.). All Macs, iPads and iPhones are not DDM enabled.

Where do you see that it is not enabled? 

In the Software Updates tab, is the "Use new feature" on? Have you tried sending out a DDM Scheduled update command to some test devices yet?

yes, we tried that and it is not enabled and the command failed. In every Single device it says "Declarative Device Management not enabled", even our Macs. very Strange

That is strange. Paper says that Jamf Pro enables automatically. Might be worth a support ticket.
Let us know what you find.
https://learn.jamf.com/en-US/bundle/jamf-pro-documentation-current/page/Declarative_Device_Management.html

Ticket is open, I'll let you know what the solution was

Any feedback from the ticket? I am also encountering the same. We meet all requirements, but DDM is not being enabled on devices. 

Yes, you have to contact Jamf Support and they have to reset something in your environment. It took about 2-3 weeks, but in the end they solved it. It is nothing you can fix on your own.