Header layout

This issue is cause by The Post Grid plugin that you are using. Check the screenshot https://imgur.com/a/mLY0LST
The width of div #container should be 100%. Check if there is a setting in plugin to disable altering the width of main container. If not you can try to override this by adding following css snippet in the Customizer -> Additional CSS:

#container {
    width: 100% !important;
    padding: 0 !important;
}