After several failed attempts to create Ubuntu-Mate boot sd card from my Ubuntu machine, I decided to try to create one from my Windows box. I reformatted my sd card with SDFormatter and wrote the image file to it with Win32DiskImager.
I decided to try this after re-reading the Raspberry Pi image installation guides.
“https://www.raspberrypi.org/documentation/installation/installing-images/README.md”
This worked like a charm for me and I hope it will work for you too.