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I found a Microsoft thread for a similar issue. Please try the suggestion provided by a user HeikkiLaukkanen and check if it works. Copying his comments:
For those on Windows 8, 8.1 and 10 YOU NEED TO TURN OFF SECURE BOOT OR YOU CANNOT TURN OFF DEP!
once you have turned off DEP you can then turn Secure boot back on again and you will not have DEP
Be sure to disable the Secure Boot Policy
Go to Settings / Computer Settings / General / Advance Start-Up's Restart Now / Troubleshoot / Advanced Options / UEFI Firmware Settings / Restart / Computer Setup / Security / Secure Boot Config / Accept / Secure Boot / *click to disable* / File / Save Changes & Exit
Reference: https://answers.microsoft.com/en-us/windows/for...
Let us know if this helps you.