ISDone.dll - An error occurred while unpacking! Unarc.dll returned an error code: -1/-12

These errors may be caused by a missing version of Visual C++ on your computer. Therefore, install the common redistributables.

Additionally, it is recommended to create a folder to install your games, add an exclusion to it, and select it in the setup:

After that, start the installation again.

If it is a FitGirl game, also activate the RAM memory limiter at the beginning of the installation setup:

If the error persists, install the game from another source that makes its games available in a pre-installed or portable format.