Hi @Seerat Ch ,
Same as you, I'm in the situation before. It looks like the permissions assigned to the Organization Management role were a bit messed up. Based on your description, You're most likely missing Mail Recipients or Mail Recipients Creation. To resolve this issue, you need to follow the steps below:
-Login to EAC as Global Admin.
-Select the Roles tab.
-Click the Admin roles tab.
-Choose the Organization Management.
-Switch to Permission tab.
-Find the Mail Recipients and Mail Recipients Creation and enable it.
Note: after you save the changes, you might need to sign out and close the browsers; It may take a while for these changes to work. Then you can add alias for users successfully.