You've got the error properly
diagnosed. Either your SP's metadata isn't stored in the
idp-metadata.xml file you're pointing at(which wouldn't surprise me one
bit), or your SP's entityID is improperly configured(less likely).
I would simply say that you need to get the metadata about the SP, stick it
in the IdP's metadata folder, and then look at that doc page or the examples
in the relying-party file to see how to load it.