[Excel] How to make a bubble heatmap

Excel can’t encode color gradients, but you can use bubble size to create a passable heatmap.

Much of the effectiveness of heatmaps resides on how the categorical axes are sorted. Try to sort them (manually, if needed) in a way that similarly sized bubbles are clustered together.
Check for outliers in the variable you use for bubble size
To display the center of the bubble, add the series twice, but use a small and fixed value for bubble size.
Check the aspect ratio.
Reduce opacity to avoid occlusion (larger bubbles hiding smaller ones).
Show bubble outline using a darker color.