Prerequisites: You have set up an electronics cooling study in the Simulation workspace and you have modeled a heat sink in your design.
On the Setup toolbar, in the Cooling panel, click Heat Sink to open the Heat Sink dialog.
In the canvas window select the heat sink, and confirm it is highlighted in blue in the canvas and that 1 Body is selected in the Heat Sink dialog.
If you have multiple heat sinks, select them all and confirm they are selected in the Heat Sink dialog.
Click OK to accept the selection and close the dialog.