Unclear error message in Import

Hello Community,

While creating an import definition and attempting to use it in an import, I get the following error message. Now there is a reasonable chance that I made a mistake in the configuration, but I don’t understand what the error message is trying to tell me.
Has anyone seen this error before, and can point me to where I can fix it?

I figured out what the problem was, I had a key value field that was undefined