Can't enable Membership - "You do not have sufficient permissions to access.."

Hey, I installed Membership on my Wordpress site. When I try to Enable it I get an error:
"You do not have sufficient permissions to access this page".

I am the admin for this single Wordpress website.

I tried:
- Using a different theme.
- Disabling all plugins.
- Re-installing WMPU DEV and Membership.

I searched through every similar thread for a solution but couldn't find anything.

Please help, thanks.