The Working dialogue indicates that a least-squares fit is in progress and reminds you that you can not change the model while the fit is in progress. The dialogue will be dismissed automatically when the fit is complete.
Click on OK to dismiss the dialogue or click on Cancel to stop the fit at the earliest possible opportunity.
Note that the fitting process is computationally intensive and the SLIME will not be able to respond to input immediately while it is going on: this includes providing the usual feedback that shows that a button has been pressed.