Welcome to Android Central! It would be helpful to see your
Settings>Storage screen. Internal Storage on older phones is often divided into an App Storage partition and a separate Internal Storage (also referred to as "Internal SD") partition. The App Storage partition is where apps are downloaded and installed; since this partition can be somewhat small, it can get filled up easily. You might still see a lot of space in the Internal Storage, but this may be acting like an SD card, in which case you can't download and install apps directly to this area. You can't directly download apps to an external SD card.
See this guide for more info:
http://forums.androidcentral.com/amb...-warnings.html
