[Tex/LaTex] Package keyval Error: letter undefined

Question

I have no idea what's causing this, I have about 1800 lines of code, and no idea where the problem is. Could someone explain what the keyval package is, and what this error could possibly mean?

Answer 1

Found the problem, had

\usepackage[letter, margin=1.5in, top=1.5in, bottom=1.5in]{geometry}

Removing "letter" from the package options fixed it