Does that mean I have to subscribe a test user first, and then navigate with that test user to the protected folder?

If no one is subscribed to the plan yet then yes you need to subscribe someone for it to generate the files. Navigating to the folder does not generate them.

assumed that it was enough that the test user was already subscribed, and only had to log in and navigate to that folder.

If a user is already subscribed to the plan then yes all they have to do is login to generate the files.

Seams to be working fine in my tests. See the below for examples.

Outside Joomla Root (live)
Path: /user/restricted
Type: Absolute

Outside Joomla Root (localhost)
Path: /server/www/restricted
Type: Absolute

Inside Joomla Root
Path: /images/restricted
Type: Joomla

All 3 test cases created their htaccess files and respective htpasswd files on login. I don't know what is blocking your site from creating htaccess files, but my best guess is permissions problem or wrong path. I did confirm for Absolute usage the status check always says "No path defined yet. Please define then apply setting to get result of check." even if the path is correct and have added a bug ticket for that.

forge.joomlapolis.com/issues/7721

Ok, was able to find a bug when using more than 1 folder access within a single plan. The first folder access seams to always work fine, but the second can break when performing the folder access loop to check all 5 available folder access paths.

forge.joomlapolis.com/issues/7722

Both issues now fixed. Fix is awaiting review followed by merge then release. Everything should work perfectly fine as long as you only have 1 path configured though.