Seems your other directories are owned by root rather than www-data, which is NextcloudPi user. As from this guide:
If you see no errors you should now be able to access /mnt/directory/location on the local file system. Now we will change ownership:group to the data in this location from root to www-data since that is used by Nextcloud in Docker. See current permissions with `$ ls -la /mnt/directory/location
$ sudo chown www-data:www-data -R /mnt/directory/location