In the below image of a Bar Chart, "Side-by-side bars" has been selected for the Layout , but the plot's bars are displayed as Stacked bars.
This can happen when the Bar Chart's coloring mode has been set to "Continuous" instead of "Categorical".
In the below reference image, the "Advanced Settings" dialog for the plot's Colors property shows its "Axis mode" set to "Continuous".
Once this is updated to "Categorical" as per the below screenshot, the bars are displayed as Side-by-side bars.