the dd command needs sudo dd in front. so your example would be:
sudo dd if=~/Downloads/ubuntu-mate-18.04.3-desktop-i386(1).iso of=/dev/sdb status=progress
then sync would be a separate command after that one is finished.
if you want sync in the same line , you could add && sync for:
sudo dd if=~/Downloads/ubuntu-mate-18.04.3-desktop-i386(1).iso of=/dev/sdb status=progress && sync
&& tells bash to run the second command only if the first completes successfully.
unless you were asking about the ‘attribute “partition” not found’ outupt? since i usually use gparted to make my partitions and file systems, i wasn’t familiar with that but it didn’t look like a good thing. this thread suggests you don’t need to worry (i never did bother doing so either. just wanted to see some written confirmation) about formatting the drive before dd.