File Browser is a simple tool to access the file system without enabling developer mode.
Your storage (phone or SD card) is most likely full.
One reason for this error message can be that you tried to move a folder between the phone and the SD Card. Instead of moving (cut-paste), you should copy the folder. After successfully copying your files, you can delete the original files.
It means that the xdg-open command used to open files does not recognize
the file type or it doesn't find a preferred program to open it. You can use
Mimer (in OpenRepos) to
configure preferred programs.
Perhaps your SD Card is not properly inserted to the phone. Maybe it has a file system which is not recognized by the phone.
You can also try to access the SD card with another file manager, such as Cargo Dock. Please get in touch if it works with a different file manager but not with File Browser.
You need to enable installation of untrusted software in the phone. Go to Settings -> System -> Untrusted software, and enable "Allow untrusted software".
Tap on the file or folder icons in the list view and you will see options to cut or copy at the bottom of the page.
You can also press and hold the file or folder icon until it becomes bigger. Now you can tap on another icon to select all files in between.
It can't change owners, edit files, send email, MMS or SMS, open ftp sites, connect to samba server, simulate vi and it does not blend or make coffee. Sorry.
It can however share files, preview PDF files, and directly open system storage settings when using the version from OpenRepos. These features are not allowed in Jolla store.
You can also install root mode for File Browser from OpenRepos. Beware: this allows you to easily brick your phone!
There is the wonderful Cargo Dock file manager in the Jolla store.
There are a number of other file managers available in OpenRepos.