Can you provide more details on your issue? Are you using GOG Galaxy? Are you using the backup installers? What was the exact description of the error that occurred?
If you are using GOG Galaxy, try reinstalling the client and redownload the game.
If you are using backup installers, try redownloading the installation files and check their integrity before installing the game.
Thanks for your response,
I use GOG Galaxy and i started downloading with GOG Galaxy, when i have the error i have just " error : installation failed". If i reinstall the client i have to redownload the game ? I have a bad connection (1.5mb/s) it's too long.
PS : i have the same problem with the Witcher 3 installation
Hi again Nashoba68.
When using GOG Galaxy to Install a game, after pressing the "Install" button, a pop up window appears, which allows you to choose the destination path of the folder, where you want the game to be installed. After you get the "installation failed" message take a look whether the folder mentioned above has any game files, of meaningful size, in it. It is possible that a "failed installation" deletes/removes all of the files already downloaded.
If you manually halt an installation, then the already downloaded files are removed anyways.
It is very important that the installation folder you choose, has enough free space for the game to be stored.
In case you reinstall GOG Galaxy, the folder containing your installed games will remain unaffected, as well as any incomplete installations. After reinstalling Galaxy, the program should be able to recognize any progress made on incomplete installations, as long as the files are there, and continue where it left off.
Alternatively to using GOG Galaxy (which is optional) for your game downloads, you can use the browser based GOG.com site or their "GOG Downloader" program (which has the fastest download rates), to download backup installers of the games, which can be imported on GOG Galaxy, after their installation.
If you have any further questions, please let me know.