You need to select a folder to install MTA in, then you need to select a folder where your gta_sa.exe is located, (usually C:/program files/rockstar games/gta san andreas/), if it doesn't let you proceed then it cannot find gta_sa.exe meaning it isn't the right folder. It could be that you're out of space too, but I highly doubt that. Are you running the installer as a user? Try running it as administrator.