Smile [Q] I want to install a CM11 Nightly, but the flash always fails.

I have flashed ROMs a few times before on a couple different devices, but I have a couple questions.

1. When I try to flash a OnePlus One CM11 Nightly through TWRP, it always says "Failed". Is there some simple thing that I am forgetting to do?

2. Will it erase anything that is currently on my phone by flashing a nightly? (I don't care if I have to re-root, but I don't want it to erase anything)