Originally Posted by sidneyk
assert(getprop("ro.product.device") == "tf101g" || getprop("ro.build.product") == "tf101g");
format("ext4", "EMMC", "/dev/block/mmcblk0p1", "0", "/system");
mount("ext4", "EMMC", "/dev/block/mmcblk0p1", "/system");
The problem is that TWRP does not mount the /system partition by default and the updater-script is trying to check for keys in a file that aren't yet visible to the recovery system. The script is only mounting the /system partition after it has tried this check and after it has already formatted /system, which, of course, fails because there is no build.prop file now to even check.
You can modify the updater script (in a non-windows text editor) and move the mount line (line 9) above the assert line (line 7) or just delete the assert line altogether. I would do the second option, if it were me. Then just zip it back up, create another md5 file if you want and try again. Just make sure you're installing to a tf101g, which is what the ROM was built for.
The ro.product.device and ro.build.product are set by TWRP when you are running recovery. My guess would rather be that you are not running a recovery compiled specifically for TF101G, but rather for TF101. Then the above assert would fail. The way to remedy it would be to first flash the TWRP for TF101G, reboot to system, reboot to recovery again, and then try to reflash the ROM. You can check in TWRP Advanced menu, "Run terminal command":
getprop | grep product
Most probably this will give tf101, not tf101g