I have the impression you need to enable aggregations to have access to the Relay connections of a table but schema introspection would make the connections visible anyway for all the tables/views that reference it, which creates a runtime error:

{
  "errors": [
    {
      "extensions": {
        "path": "$.selectionSet.node",
        "code": "unexpected"
      },
      "message": "could not lookup \"foo_connection\" in 'bar_table'"
    }
  ]
}

First I think it would be good to document the fact you need to enable aggregations to access connections, and secondly the fact that the connection looks available in the schema but is not fetchable looks like a bug to me.

I tested this on 1.3.0-beta.2 and 1.3.0-beta.3.