Ok today's fix is faced by a lot of people on all iDevices (iPhone, iPad, and iPod) this problem occurs sometimes when you try to update an app through the app store and although you enter the correct credentials still you get this problem.
1- That this problem is caused by carrier restrictions.
2- That this problem is caused by 3G or WiFi.
3- That this problem is caused by the latest update or backup you made.
4- That this problem is caused by the jailbreak or unlock.
1- Might be caused by a corrupt restore after update.
2- Might be caused by playing with the root size of the device (jailbreak specific)
3- Might be caused by caused by small root folder size specified during jailbreak
4- Might just occur because of nothing.
Always make sure that your have a valid connection to the Internet.
If you don't have a jailbroken iPhone,iPad, or iPod I fear that your chances at fixing this are limited, as you have to try the following:
1- Try resyncing with iTunes
2- Try Signing out then in on your appstore account.
3- Try disbaling/enabling the internet whether through 3G or WiFi.
If the above failed you can:
1- download and update through iTunes
2- Remove the application and redownload it as some applications my have gotten corrupted between updates of the firmware. (you will lose all saved settings doing this)
3- Retry updating more than once.
4- if all above failed and choice one is just too much of a bother, you can do a full restore, that will always fix it, start up as clean.
If you have a Jailbroken iPhone,iPad, or iPod you can do all the choices mentioned above and there is another method that will fix this problem but you will lose the icons arrangements on your springboard.
1- Go to cydia and get iFile the free version will do just fine.
2- Open iFile and Navigate to /var/mobile/Library/Caches
3- Copy com.apple.mobile.installation.plist to another folder or just rename it. (this is to make a backup)
4- Delete com.apple.mobile.installation.plist
5- Reboot your iPhone or reset it by holding power+home till it restarts.
6- Once you see all your icons on the springboard.
7- Try updating again.
Sometimes the icons take a while to repopulate the backup is taken just incase they don't populate again. also you can use SSH to do the same.