It seems you've missed a file. I think the missed file is:
Take a look into your code to see if that file is present (if not, you can download it from GitHub)
I can clear the error by disabling the compiler, but it does introduce a security warning.
Important: Formkey validation on checkout disabled. This may expose security risks. We strongly recommend to Enable Form Key Validation On Checkout in Admin / Security Section, for protect your own checkout process.