Default TWRP won't flash CM 11

I have downloaded the CM11 package from the Cyanogenmod website and the Gapps from goo.im. Whenever I try to flash any of them, I get an error with E: and whatever usually comes after that with Failed in red. Does it have something to do with being on rooted stock KitKat? If you can help, thanks. The stock lags badly and the battery drains fast.