I have one tablet and I'm installing a
.zip file from stock recovery, but it gives the following errors:
Opening update package... Verifying update package... Verification failed Installation aborted.
What causes this type of errors?
It sounds like the bootloader on your tablet is still locked. By default, most Android tablets will only boot and run stock recovery images and ROMs, for security. It verifies that the ROM is signed by the original manufacturer (or the carrier, for carrier-branded devices). You need to unlock the bootloader to let it know you want to run an unofficial image.
See the bootloader-lock tag for more details, and a starting point to find out how to unlock the bootloader on various devices.
You usually cannot install anything but the official updates using the stock recovery. It checks signatures of the update files, for a good reason. Without that, anyone being able to modify the ZIP file (during download or on the storage, which can be available to ordinary apps) could get a root access and modify anything. It has not been intended for installing custom packages that are not signed by the manufacturer.
You need to use a custom recovery that is free of this restriction. It depends on your phone/tablet.