First of all make sure that you launched game via GOG Galaxy and you meet all of achievement conditions.
Also please add game files and following Galaxy files to both your antivirus and firewall trusted/exception list:
C:\Program Files (x86)\GOG Galaxy\GalaxyClient.exe
C:\Program Files (x86)\GOG Galaxy\GalaxyClientService.exe
Make sure to restart GOG Galaxy, after following above and try to unlock problematic achievement once again.
Note that the achievement sync may take some time, so please wait up to 24 hours and check once again if your achievement has been earned.