The documentation explains, “Alerts can only be set on tiles pinned from report visuals, and only on gauges, KPIs, and cards.” In other words, alerts are only available on gauges, KPIs and cards that are included on dashboards. For example, I have this dashboard and there is no option to set an alert on the bar chart:
But it is possible with the Total Shipping Cost card:
Select Add alert rule
Set Above or Below threshold, then click Save and close.
The “send me an email” option is checked by default:
The same steps will allow you to see and update your alerts: