Reply To: Unabel to change Checkout Page Columns

Try to add the following code snippet to the Custom CSS for desktop devices area in Theme Settings

form.checkout>.col-sm-6:first-child {
   flex: 1 1 66%;
   max-width: 66%;
   width: 66%;
}

form.checkout>.col-sm-6:last-child {
   flex: 1 1 34%;
   max-width: 34%;
   width: 34%;
}

And this part for tablet and mobile devices

form.checkout>.col-sm-6:first-child,
form.checkout>.col-sm-6:last-child {
   flex: 1 1 100%;
   max-width: 100%;
   width: 100%;    
}