This problem could be caused by a few reasons:
1) You are trying to update theme on the different domain that theme was activated (for example you activated the theme on localhost and moved it to live domain). Then you just need to go to Theme Options > deactivate theme and activate it using your purchase code again
2) There is no enough space on your server and archive that was downloaded for the update is broken. Make sure that you have no less than 20MB of the free space on your hosting.