Same error was posted before:
After reading about that before, it may not be specifically related to “windows input simulator” which I mentioned in response to that post, but it could be SambaPOS uses the same methods to simulate keyboard input and something in Windows is stopping it working properly. Like others suggested, make sure Windows 7 has latest service pack and all windows updates. That other post is the only time this has been mentioned on the forum, and my suggestion was marked as the solution by the original poster, so I think pretty much it is related to your issue.
SambaPOS has thousands of users globally and many using Windows 7, this will not be a problem with SambaPOS. Just because SambaPOS shows the error does not mean it is a SambaPOS issue. I have 5.2.7 running on a Windows 7 Pro system I use for demo, it works fine and I don’t get this error.