Check the device type of the heatmap
If you are viewing a "desktop" heatmap for a page that only mobile viewers can see, then you will have no data to analyze. Change the device type from the dropdown in the top-right of the heatmap.
Check that there is data for that page
You may be trying to view data for a page which has no recorded visits for the selected date range. Try going back to the heatmap list and checking that the page has been recorded in that date-range.