sudo apt-get install ack-grep
gives
ack-grep is already the newest version (2.22-1)
but ack-grep gives
ack-grep: command not found
It was working fine in Ubuntu 16.04, before upgrading to 18.04
ack-grep is a meta package to pull ack in 18.04.
Try to do sudo apt install ack and execute ack instead of ack-grep ; that's a proper command.