Are you able to download games directly off the website? The files are hosted by the same servers.
As for the app, it’s hard to say specifically what might be going on, as it’s your computer and we can’t predict what other software you’re running or how you’ve configured things. (Virus software, permissions, customization can all cause issues, among other things)
If possible, you can try a complete uninstall along with deleting the itch folder in your appdata folder. (note this will remove all downloaded games, all settings and configuration, etc. Some games may save in their install folder so if you need to backup anything do that first.)
- Windows:
%APPDATA%/itch/ - Linux:
$XDG_CONFIG/.config/itch/ - macOS:
~/Library/Application Support/itch/