In Excel do a 'Help' search for 'Add a Watermark' and you will see there is no button for adding a watermark but the Help item shows how to do this via Headers/Footers.

Watermarks are much easier to insert if you could use Word instead of Excel to create whatever it is you are creating. This watermark would be a 'picture' of a vertical rectangle which would be the size of the border you need. It would appear on every page which fulfills your aim.
This Watermark insertion is done via Word's 'Design' tab / Watermark / Custom Watermark / Picture Watermark / Then select 'From a file' to select the picture of a vertical rectangle that you would have already created. Untick the 'Washout' box to make the border more visible.