Why is the build cutting off the sides of my game?

Despite the camera working fine when playing my game in the editor, when I build the game, the sides get cut off! I have tried using full screen, but it does not fix the problem. Does anyone know what is going on?

Thank you for your help!

In the build settings, before clicking on the build option, go into the PlayerSettings and check the circled areas in my picture and compare them to yours. You could have unchecked a supported Aspect Ratio or something in the Resolution settings.