Assuming you set everything good in your .env I think your mysql is not installed correctly
I have very same problem but on AWS deployment I think the script refers to superseded packages and mysql is not installed at all
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
mariadb-server : Depends: mariadb-server-10.1 (= 10.1.14+maria-1~xenial) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
sed: can't read /etc/mysql/my.cnf: No such file or directory
forge.sh: line 410: mysql: command not found
forge.sh: line 411: mysql: command not found
mysql: unrecognized service
forge.sh: line 414: mysql: command not found
forge.sh: line 415: mysql: command not found
forge.sh: line 416: mysql: command not found
forge.sh: line 417: mysql: command not found
forge.sh: line 421: /etc/mysql/my.cnf: No such file or directory
forge.sh: line 422: /etc/mysql/my.cnf: No such file or directory
forge.sh: line 423: /etc/mysql/my.cnf: No such file or directory
forge.sh: line 427: mysql: command not found
In my case pgsql works fine but if using mysql you will get same or simillar error
Hopefully @TaylorOtwell may have suggestion? I just add that was is still using 14.04 and maybe script was updated to 16 run currently on DO