You are drawing a feature layer in your map with heat map symbology and sharing the layer to the web. ArcGIS Pro provides two types of rendering methods for drawing heat map symbology: constant and dynamic mode. The mode is set with the Method property in the Symbology pane.
If the Method property is set to Dynamic, the layer cannot be shared. Map viewers can only draw heat maps in Constant mode.
Do one of the following:
Change the heat map's rendering method to Constant mode. Follow the steps provided in Heat map symbology to locate and change the rendering method.