I'm using the itch.io desktop app and it says "No manager for installer unknown". In the stack trace it says "no mapping for file extension .86_64". I'm running Pop!OS 64-bit:
$ lspcu Architecture: x86_64 CPU op-mode(s): 32-bit, 64-bit
Maybe you have to make the file executable? I found a site with a guide on how to do that, please let me know if it works https://linuxhint.com/make-file-executable-linux/