It took me ages to enable the files permission for emacs.
The easiest way is:
- Go into your phone's settings and search for 'all files access'
- In the Apps results subsection, there should be an option for 'All files access', tap it
- Then tap on 'All files access' in the Special access list
- There should be a list of apps with 'All files access' permissions
- Search for 'emacs'
- Enable the permission for it
That should be it.