At the top of the "Stack" module are all the buttons that control the execution of stacking in Nebulb and, specifically, the button below is used to start the stacking process of all imported images:
While the stacking process is running, this button is disabled (cannot be pressed) and displayed in orange.
And when the stacking is finished, this button is displayed in orange with a check mark. At this point the post-processing modules (right tool panel) are enabled so that the user can make any desired adjustments to the image, such as reducing light pollution.