By default the Simple Custom CSS and JS plugin creates the “web designer” role with the “*_custom_csss” capabilities and assigns it to the administrator.
There are several plugins that modify the users’ permissions, for example User Role Editor or WPFront User Role Editor. If the role or the capabilities were removed for your current user, then the permissions error will show up.
Would you please do the following: install the User Role Editor plugin, go to the “WP Admin -> Users” page and click the “Capabilities” link associated with your user (see this screenshot), make sure to enable the “Web Designer” Role and all the “*_custom_csss” capabilities and then click “Update” (see this screenshot).
You can remove the User Role Editor plugin after updating the capabilities.
