Have you tried: sudo apt-get install fuse to see if you can install the package? It's a common library missing from most distros that the AppImage needs to run. I'm not sure about the specific error you're receiving though
I’m an Arch user, so my Ubuntu experience these days is super limited. I’m not sure how to fix this issue, but there are quite a few users here that possibly could. Sorry for not being of much help here.
Any thoughts on implementing a checksum here? That would definitely help make sure the downloads are complete and right. I noticed it’s missing from the script and main download page and should be pretty simple to implement.
Joplin is great, I have recommended it to my friends. It’s really hard to download it in China because of the GFW. I spent almost 3 days to download the binary file.