Having full access to the FB page does not mean you has admin access on the ad account, which you need as well. Make sure you have admin access to the ad account and or business manager.
This is the issue faced by many of the merchant using shopify, when they connect your facebook page to shopify store.
You should once again sign in to your facebook page and check for the level access that you have.
you can check for the access level in Business Setting. The following steps helps to check the access level,
> Go in to the Business Setting
> Accounts > Pages then you see the names and id of the people you have the access to your page, click on your named id and check this level of access
> Admin Access must be checked on.
Please let me know if it helped!