Hey there! I don't have experience with the itch.io launcher. Our game actually runs from the folder, so it doesn't really require installation, and trying to install it to a read only location would cause issues.
I would suggest going to "My Library" (click the down arrow in the top right of the site next to your username to find it), and clicking the red "Download" button beneath the game name. Once downloaded, try moving the zip file to your Desktop and unzipping > moving the folder out of the zip archive and onto the desktop. Run pure-onyx.exe from that folder.
If this doesn't work or results in the same issue, can you see if a log file was created and send them to us at eromancergames@gmail.com?
Log files are located at
%USERPROFILE%\AppData\LocalLow\Eromancer\pure-onyx
You can zip the entire folder and send it to us :D.