Someone told me that in the Shulchan Aruch it says that a Jewish court can’t take a case after someone went to a secular court and didn’t win.. Is this true? if yes, where exactly in the shulhan aruch does it say this? thank you

Sueing someone in secular court is a tremendous aveirah and a big chilul Hashem. The source you look for is choshen Mishpat 26:1