need help interpreting error message

I am getting an error message I have never seen before in my account and can't see what it refers to--and I'm way past the support hours. It appears at the top of the campaign, above the section with the "From", "Reply to",  subject line and preheader, in a pink box and says: "a is undefined" with a warning triangle to its left.

Obviously it wants me to fix something but I can't figure out what "a" refers to. Can you explain what I should look for to fix?