The SAML Request AssertionConsumerServiceURL is invalid: 'https://www.dropbox.com/saml_login'

I followed the tutorial but I get the error posted below.I don’t think I’m making a mistake because the Dropbox settings are very simple, as the picture below shows.
Anyone know what step I missed?

" invalid_request : The SAML Request AssertionConsumerServiceURL is invalid: ‘https://www.dropbox.com/saml_login