there are a few preparatory steps required before an external user is able to see reports via a link.
please follow the instructions following YF forum post and let us know if that helps:
I did install a local version of Yellowfin that wasn't integrated into our website SSO and I was able to get external users to view the reports just fine.
can you please confirm that have a Multicast licence?
and also, can you confirm that you are using the webservice function LOGINNOPASSWORD