Confusing compiling error

Hi LyX Users, I have been cut/pasting source code from a LyX document to a LaTeX document, and compiling the latter without difficulty until now. Out of nowhere, after the last pasting from LyX to LaTeX, I suddenly get a compiling error which doesn't go away when I undo the pasting. The GotoErro

LyX/LaTeX compiling error

source int TeX shop and click typeset, the compiling fails and reads out the same errors.  Thank you in advance for whatever help you can offer. Sincerely,Maaneli Derakhshani.