Unable to authenticate with session_id and server_url

[ravionrails]

client = Soapforce::Client.new(client_id: Rails.application.secrets.client_id)

resp = authenticate with SAML assertion
client.authenticate(session_id: resp['access_token'], server_url: resp['instance_url'])
got response int HTML with parse error
Savon::InvalidResponseError (Unable to parse response body:)
Got following reponse https://gist.github.com/ravionrails/9cd33a595045319e847b951e4cc147e0
Although no issue with username and password