Some pages and plugins require the user to be a specific role or have specific capabilities due to security rules. This is one of the main reasons this happens while trying to show a page in the frontend.
There are some basic troubleshooting steps that you can take in order to make it work.
We suggest you install the plugin User Role Editor, it allows you to edit all the capabilities of a specific role. You could create one test role with this plugin and then give all the available capabilities.
If you can see the frontend page with this user role, that means that one of the capabilities that you added is required.
Then you can remove capabilities one by one until you find which capability is needed for the page. Please don’t keep all the capabilities in the role because you might be giving them sensitive permissions like manage options, activate plugins, etc.
Only add the one or two capabilities required by the specific page. For example, if you want subscribers to create courses, only give them the capabilities related to edit courses, create courses, delete courses. Don’t give them capabilities like manage options, activate plugins, etc. because if you give all the capabilities you would be essentially be converting them into administrators which is dangerous.