Looking for some assistance in my code. Not sure where the fault is.

Hello Friends :)

I've been re-teaching myself Arduino to brush up on my C/C++ and I've been working on this project for a simple adding machine with two user inputs and a single visual output. I'm not sure what is wrong with my code.

As it stands, the code compiles without error or warning and runs on my Uno R3, but as soon as I press "NumberEntryButton" to begin building the first number, pin 13 immediately flashes 8 times, then stays solid on.

If anyone has advice or help to offer, I'd super appreciate it.

I've attached a permalink to the file in my git repo.