To identify why you are unable to edit a security group in Active Directory, please follow the steps below:

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1. Check if you’re a member of the right AD security group, since its a group-level permission to edit security group.

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2. Ensure that you have given the proper rights/permissions of 'Edit Security' to the Active Directory group.

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3. If still you are unable to edit, then check the group policy settings which are applied at domain and OU levels.

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4. Check the delegation of permissions at account level and at the OU level.

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5. If the issue still persists then you may need to adjust the permissions for the entire domain.