The OTA update script used to upgrade from one version of Android to another has been changed. Now, rather than checking your android device and updating individual files, the script looks at an android device's system partition as a whole. This, as Google Android Police points out, means that anyone who has rooted or modified their Android smartphones or android tablet in any way will find that the OTA update will fail.
However, this does not mean that all is lost. While slightly annoying, if you have modified your android latest operating system partition all you need to do is reflash your android device with a stock Android image. With this done you can then reattempt the OTA update and it should be successful.
Another option is to manually upgrade android latest google nexus 7 system version to Lollipop. Factory images for Android Lollipop are available for some Nexus OTA devices – we've looked at how you can install the update on an OTA Nexus 5 4.4.4 and how to upgrade a Nexus 7 OTA and images will be made available for other handsets in the coming year.