Can you give some background on this screenshot?
400 status code received on POST method for API /api/v1/stacks/HDP/versions/2.6/recommendations
Can you share your /var/log/ambari-server/ambari-server.log
According to the screenshot you setting up a new cluster, are you using a default derby database or you created one ? If so did you choose the right database?
Can you check the permissions of the below directory make sure its owned by root or the one running ambari
# ll -d /var/run/ambari-server/