Problem with Ubuntu-mate Error “Temporary Failure resolving “ports.ubuntu.com’

I have a Raspberry Pi 3 Model B. I have downloaded and installed ubuntu-mate-18.04.2-beta1-desktop-armhf+raspi-ext4.img. When I try to sudo apt-get update I get the error Temporary Failure resolving ‘ports.ubuntu.com’ The raspberry pi has an ethernet connection to the internet. I have searched many times to find a solution but nothing seems to work. Is this…

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