If the error that leaves you is the 491 to download heavy applications such as some games that weigh more than 50 Megas, chances are you'll almost fills the internal memory of the device.
Sometimes we are confident in knowing that our device has 500 MB or more of free internal memory, but you have a small detail in mind: Android applications are downloaded APK files, which are nothing more than ZIP files renamed, and to settle, Android has to unzip the contents to a temporary folder, and then copy those files to their final location. And these unzipped files can peacefully occupy a 400% increase, especially if it comes to games. Example:
If you have 500 MB free and the application we want is a game that weighs 100 MB, it is highly likely that you cannot install it ... because when decompressing files the application can take leisurely triple (300 MB), and then to install needs copy those 300 MB to the destination folder, so you would be missing 500-100-300-300 = -200 MB of space needed ... that is at least 700 NASA FREE (500 + 200).
So the heavier applications work best in best tablets or smartphones, simply because they have the convenience of having more internal memory to work. On the other side, to give an example, a smartphone with Android 4 and only 512 MB of RAM is not a good place to run a game high caliber, because if you think about such a heavy game, will have heavy graphics and If you already have several applications installed RAM and occupying only would slow your device ... but is unfortunately still many manufacturers who still cheat RAM and internal memory to lower its price.
But resolving the error 491.
First you should check how much free space is left in the internal memory and microSD. In Settings, Storage, we will show how each is free. If you really have less than 50% free (if for example you have 512 total MB and less than 256 MB free), you should try to move applications to external storage: Settings, Applications, see all you've installed and move all which can to external storage card (microSD): you play the name of each application to enter check your installation data, and whether the system and the application will allow, you'll see a button that says "Move to SD card "; just be aware that according to the weight of the applications may take a few seconds to move. Then we will compare how internal storage had before and after this move and so you know if it was worth it. The apps run faster in internal memory, but at the cost of staying out of space quickly to continue installing (that's the fine print of Android).
The litmus test to test this is to try to download an ultra-light application, weighing less than 1 MB, and you should have no problems with that, because the 491 is when the installation fails because of low free space.