Url in button not working

This is the url I am trialling in a button

https://quatresaisonscreations.com/?modal-link=https://quatresaisonscreations.com/tadelakt-2/modal-content

Not working – I just get a blank white window with nothing in it but a close cross.

I haven’t changed any plugin settings nor done anything to the tadelakt2 page I want to popup.

What have I missed?