Check this out Displaying Graphics in App Designer
Grid layout managers and scrollable containers do not support axes, polaraxes, geoaxes, or charts (such as heatmap, geobubble, or stackedplot) that can be the child of a figure
You can display it on figure not on the axes. But you want to display it on the UIaxes. I would recommend to contact MATLAB support.
cdata = [45 60 32; 43 54 76; 32 94 68; 23 95 58];
xvalues = {'Small','Medium','Large'};
yvalues = {'Green','Red','Blue','Gray'};
heatmap(app.UIFigure,xvalues,yvalues,cdata)
