I have successfully set up a Kibana 4.x dashboard along with Elasticsearch 2.x, but I feel 5.x versions should be the future-proof way to go. However for some reason Kibana
I had exactly the same problem. After looking everywhere on the internet, desperate, I came back to Kibana > Management > Index Patterns and clicked the orange button (refresh field list). I noticed that the columns in the "Fields" table updated, especially "aggregatable". Back to "Visualize", I celebrated with a well deserved coffee that "Field" field of "Sum" Aggregation finally accepted to discover my number typed field.
P.S. At the moment I am still struggling to have a proper graph... (I do have a table though).
Edit: Actually I was using an index pattern that was created with the "Index contains time-based events" checkbox unchecked. That is why I was experiencing this issue: https://github.com/elastic/kibana/issues/9458