Can't get report column to work

I have a report that views the info from three sheets.  All three sheets contain a "Estimated Completion Date" column, but for some unknown reason, I can't include that column in the "What?" section of Report Builder.  I would like to configure the report such that it only shows records where the "Estimated Completion Date" column is not "TBD", but when I click on the "What?" button, and it prompts to me to select a column to add to my report, the "Estimated Completion Date" column isn't listed.

How can I accomplish my goal?