What does error message 'Process aborted: this tool is not ready', indicate when doing an analyze or build in Unity Pro?

Goals and Symptoms

What does the error message "Process aborted: this tool is not ready." indicate when doing an analyze or build in Unity Pro?

Causes and Fixes

This error message indicates that a modification has been made to the project but it has not been validated. The text ahead of this message in the output window will indicate where the modification has been made. Below is a screen capture of the error, with a change made to the configuration of an analog module.

In order to validate the modification, you have to switch back to the window where the modification was made and select the check mark icon in the tool bar to validate and confirm the change, or close the window and accept the dialogue box prompt to 'Save Modification'. Now an analyze or build can be performed without error.(If you have made many changes, you can simply select 'Windows -> Close all Windows' and you will be prompted to validate as needed while Windows close)