Why is the Once Per Bar option missing?

The Trigger parameter determines how often and when exactly the alert will be triggered. 

The Trigger section can consist of two or four frequency options.

All four options (Only Once, Once Per Bar, Once Per Bar Close, Once Per Minute) are only available for interval-dependent alerts – i.e., based on drawings, indicators and chart types such as Heikin Ashi or Range.

Two options are available for regular price-based alerts - Only Once and Every Time.

The price value is identical on all intervals, and such alerts are triggered regardless of the interval. In case you’d like to set up an interval dependent alert, you can create a horizontal line alert.

Strategy alerts are triggered on every order. The triggering frequency for such alerts is determined in the strategy code, so no frequency options are available in the dialog.