I tried many times how to boot with Ubuntu-mate on my raspberry pi 3 B+. I always get the same result. I have created a picture on the screen. To access it, please use https://drive.google.com/file/d/1hJFzCK-Y7r9V8te5Be_ATIiDr1RdrQuc/view?usp=sharing.
Does it mean something to somebody? Thanks.