iMovie still will not download, despite having no other apps, and ~2 GB of storage available?
According to the App Store, iMovie would require 641 MB of storage to download on my iPhone 5S (it should be about the same for a 5C), so yes, you have more than enough storage available. I'd try restarting the iPhone, accomplished by holding the power button and home button down together until you see the Apple logo appear. It is completely safe, I do it to my iPhone all the time, it will not delete anything off of your iPhone, nor change any settings, but it may fix little issues like this.

If that doesn't work, try downloading the app through iTunes on your MacBook (or PC) and then sync iMovie to iPhone, that requires less storage, anyway.