First download zenmap -> "Optional Zenmap GUI (all platforms): zenmap-7.80-1.noarch.rpm" from https://nmap.org/download.html
Should be in your downloads folder.
Then run these commands:
apt-get update
apt-get install alien
Go to your downloads folder cd Downloads
sudo alien "name of downloaded package.rpm"
sudo dpkg -i "name of converted package.deb"
zenmap