>> I am logged in as Admin, and working in my DAP dashboard in WP no problem. Now that I have protected a page, why am I getting the error message :
Sorry, you do not have access to this content.As the admin...
Shouldn't I be able to view the pages I protect, no matter what the product and without having to log-in as (or add myself) a paid user? <<

Admin user does not have default access to protected pages. We do not allow default access because we had users open tickets saying 'content protection is not working' without realizing they were logged in as admin to dap while testing content protection. So we decided to not give default product access to admin users.

You can give dap admin access to products in DAP manage users page. Select the admin user row and click on "Give Selected User(s) Access to the Product..." under operations to give admin access to products.