The MicroSD has nothing to do with apps as its only purpose is to store your pictures and music. The external storage referred to is actually part of the device's internal memory but called external as it is external to the system space on these devices.
The error here is pretty self-expanatory. If you have other games on the tablet, they've likely taken up all the space. Uninstall the ones you don't play on a regular basis and try again.