On 0.2.1, trying to INSTALL_GAMES with 0.2.1 gives me this error.

INSTALL_DEMOS seems to work fine, though.

Might this be connected to the 0.2.1 release notes saying:
"Fixed: Crash when INSTALL_GAMES / INSTALL_DEMOS without a writeable disk"?

Platform: macOS Catalina, 10.15.5

yes, it was indeed! Thanks @leissler -- fixed in 0.2.1b which is up now.