That first argument needed to be post object, not post id. and it worked!
Thanks so much for the quick response!!
this “worked” in that it echoed out something… but it just echoed “content” on all the posts
I do have the custom excerpt setting checked in the back-end.
if ( ! empty( $_GET['_sf_s'] ) ) {
$thecontent = relevanssi_do_excerpt( $postid, $_GET['_sf_s'] );
}