In ArcGIS Pro, the legend bar symbol fails to display in the Chart properties pane while creating the bar chart if the selected split field has been used to apply the unique value symbology on the feature layer.
Duplicate of BUG-000116286: Currently as designed, if the bars can match the map there is no option to manually change the bar colors. We are adding the ability to manually change bar colors when the chart can match the map in a future release.