Alert settings in Looker (categorical alerts)

[kadabra]

Hi

I want to set an alert on a field in a table visualization of a dashboard. 

When clicking on the alert icon of the table, on the Condition screen, I only get one field available in the condition field drop down, which is the last column of the table.

However, I need to set an alert on another column.

Why other columns are not available in the Condition settings, how to make them available/selectable? Is there anything needs to be set up in the underlying model in LookML?