But this is just the space reserved for the binding line. For example, if the left margin of the page is 2 cm and the binding line is 2 cm, the left margin of the actual page is 4 cm.
To really set the binding line, you can set it like this:
Menu bar-View-Header and Footer-Header and Footer Toolbar appears-Page Toolbar-Drawing-Click "Line" and then draw a vertical line on the left side of the binding line, or insert a vertical text box to enter content-Close the header and footer toolbar.