apt install vsftpd -y useradd ftpuser passwd ftpuser echo "ftpuser" | tee -a /etc/vsftpd.userlist
It is not advisable to use FTP without TLS/SSL/FTPS over the internet because the FTP protocol does not encrypt passwords. If you do need to transfer files over FTP, consider the use of virtual users (same system users but with non system passwords) or TLS/SSL/FTPS.
/etc/vsftpd.conf
userlist_enable=YES userlist_file=/etc/vsftpd.userlist userlist_deny=NO
https://security.appspot.com/vsftpd/vsftpd_conf.html
https://www.digitalocean.com/community/tutorials/how-to-set-up-vsftpd-for-a-user-s-directory-on-ubuntu-16-04
https://ubuntuforums.org/showthread.php?t=518293