I am not able to reproduce this in a sample model. Try right-clicking on the column header for the bulk reaction rate and seeing this allows you to enter a value.
If you are enter a very small value, something smaller than the precision of this field, you would need to increase the precision using units and formatting.
To add to Scott's answer - if you want to use a bulk reaction rate for ALL elements, you would enter this in the Constituent alternative. The "local" bulk and wall reaction rates for each pipe and separate from the global settings - they enable you to override and enter per-pipe values. If you do not have the box checked for "use local", then it will use the global setting.
I would suggest reviewing our training material on constituent modeling such as these:
Introduction to Water Quality modeling
WaterGEMS Advanced Part 9: Water quality modelling
WaterGEMS Advanced Part 10: Workshop 5 (Water quality modelling)
Here is an article that lists other learning resources: Learning Resource Guide for new users of Bentley WaterGEMS and WaterCAD
Jesse DringoliTechnical Support Manager, OpenFlows ProductsBentley Communities Site AdministratorBentley Systems, Inc.