Hey. Yeah, this issue can sometime occur if the game is copy-pasted from somewhere and the system registry isn't updating.

This is a known issue, and the developers have an official serial key for this:
https://steamcommunity.com/app/39540/discussions/0/1694922980034570943

4Y7YP-5E7U3-2UWR5-GT0KM-ZDAJI

The last letter is "I" as in Italy. ;)

I recommend you write this down or have it up on another monitor, as you cannot copy-paste it into the game. It has to be typed in manually.

If the problem persists, you may have to reinstall the game to make the serial key request go away.

For GOG Galaxy build - just verify game files (at last twice in a row since Galaxy2.0 do not report results).
For offline build - yes, reinstall or manually add key.

Also, you can check what should be in registry in gog_game_xxx. info and .script files in installed game root folder.

And start the game as admin (GOG Galaxy if you use it, admin right for Galaxy will apply also for any games started from client).