"Unknown subcircuit" in LTSpiceXVII

As I said in my comment, LTSpice can't find the sub-circuit for the device U1. If you look at the text in the box, it says XU1, which is the ref-des given to the op-amp, the rest of text mentions the nets going in to said op-amp.

All you need to do is add ".inc opamp.sub" command as you did with ".tran 1". That solved my problem.