I get an error on a simple custom flag to tak YES to whether a task / milestone is a deliverable.

I create a Flag that simply say = Flag[14]= Yes

Then created a text filed that says   IIf(Flag14="yes",True)

The summary task is showing Yes and the task/milestone is showing error.

I want to be able to flag the task/milestone as a yes if it is a deliverable.

I am working on a stand alone version of MS Project Professional 2016

I have created such formulas before and they worked  Please HELP

Please help.