I have managed to correct all (most) of my mistakes with your help. Many thanks.
A new one popped up. If you view my site with a tablet (ipad2), and go to the CLIENTS | PROJECTS page, the three-line hamburger menu icon is missing. Or should I say invisible.
It is there, but cannot be seen. It is not white on black. So I’m guessing it is black.
The curious thing is that the remaining 3-pages are fine.
I tried making a new test page, but found the same issue. Clues? A mystery.