What can I do?

edit:though I haven't got this problem with Siberia 2 I got it again with Cyberpunk.

Guess you install from Galaxy?

I had the same problem. Install from offline installer worked for me.

I first tried to insall it without the platform because I don't want to play other GOG games at the moment but it didn't work. Because it was in 3 parts I might have done something wrong and then thought to use I used the GOG platform.
I refunded the Syberia collection and bought Cyberpunk.
Do you own Cyberpunk? Does it have a similar problem? The download couldn't start at first because I had no space, now I deleted some games but I still have an install fail problem. I will go its forum.

this is happening to me too. The only game that I cannot finish installing. Stops about 97% and asks for permission to set up on my pc which I give. Then, nada.

What offline installer would work?

It happened to me too, and I solved it with offline installer.
Inside Galaxy click the game card. Under Extras there is a section called OFFLINE BACKUP INSTALLERS.
You can also change the OS and language before downloading.