I had the same issue, but as shown above by Obligatorytoast, you need to mount something for it to work with.
I found that adding the User Apps with Read/Modify permissions for the dataset or share (depending on how granular you are with permissions) helped me.
I have found that any further work or updates to this app are unlikely to happen, but someone has taken it and spawned a potentially better app. Search github for FileBrowser Quantum.