Unable to change Group By Date Histogram to custom time term in any visualization

Using
Grafana v6.4.3,
Elastic Search data source,

Need to group by custom time field in order to see that as x-axis in Graph visualization. Tried with other visualization such as Table but has the same issue that, Group by Date Histogram shows the custom datetime field in drop-down but unable to select the same. Could anybody please help with this?