Unable to access private file?
In Drupal 8, private file permissions by default are given according to the entity to which they are attached. If the user has permission to view the entity, they will have permission to view the file.

If you are not attaching the file to an entity, then you will need to implement hook_file_download(), where you will determine whether or not the user has permission to access the file, and return a value accordingly. Currently, because you have no implemented this, access is denied by default.

Note that this hook can be implemented even when the file is attached to an entity to provide additional access checks and/or additional headers.