I know the solution.
If you are using Windows 8 or 8.1 I had this problem playing GTA on my laptop, the problem is simple and the solution is simple.
With the new SA:MP, it doesn't let you save screenshots unless you have it at your native resolution. For example, if your max screen resolution on your monitor / laptop is 1920x1080 you MUST play with 1920x1080 resolution if you want to screenshot. Otherwise it will show "Unable to save screenshot."
Simple solution is just to switch to your max / native resolution whenever you want to F8, or use a separate program to screenshot.
Hope this helps, let me know if it works.
