package:mysql-server-5.7:(not installed)
Preparing to unpack .../4-mysql-server-5.7_5.7.18-0ubuntu0.17.04.1_amd64.deb ...
Aborting downgrade from (at least) 10.1 to 5.7.
If are sure you want to downgrade to 5.7, remove the file
/var/lib/mysql/debian-*.flag and try installing again.
dpkg: error processing archive /tmp/apt-dpkg-install-CyIjq6/4-mysql-server-5.7_5.7.18-0ubuntu0.17.04.1_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1