VIDEO – "Set a mobile device fallback image for this video in your WP backend"

Hi, I have a problem with the videos which would not open on mobiles, tablets, ipads in Europe ( I checked in Spain, Poland, Germany and England). I have a web developer to check it who built the website for me and he says it works on this side – she is based in Pakistan. He advised asking here on this forum. What is seen on the video that is supposed to play is a blank screen with a message: “Please set a mobile device fallback image for this video in your WordPress backend” Can you please help?