There could be a few reasons you can't add an admin to your Facebook page. One reason might be that you don't have the admin role assigned to your profile. To assign the role, go to your Facebook personal profile and click "Manage Pages" under the "Pages" section on the left side of your screen. Then, click on the gear icon next to the page where you want to add an admin and select "Edit Page Roles." You can add additional admins by selecting their names from the list or typing in their email addresses.