I found a solution with following commands:
sudo apt-get clean
sudo apt update
sudo apt upgrade
Downgrade libc6 to version 2.31-0ubuntu9.7 because libc6-dev was in issue.
The following packages have unmet dependencies:
libc6-dev : Depends: libc6 (= 2.31-0ubuntu9.7) but 2.31-0ubuntu9.9 is to be installed
E: Unable to correct problems, you have held broken packages.
sudo apt install libc6=2.31-0ubuntu9.7 libc-bin=2.31-0ubuntu9.7
sudo apt install libc6-dev g++
sudo apt install build-essential
Pureftpd install is successful now.
