This article covers method to resolve DHCP 'isc-dhcp-server: Job failed to start' error. Basically, 'isc-dhcp-server: Job failed to start' error can happen if there is any issues with the commands that we run.
You can try to restart the service; if it really is an issue with the service starting before the network is up restarting it once the network is up should work:
$ sudo systemctl start restart isc-dhcp-server.service
If that doesn't work then try and investigate further why it's not starting by first getting the current status of the service:
$ sudo systemctl status isc-dhcp-server.service
That should also give you a PID for which you can further investigate with journaltctl where XXXX is the PID of the service:
$ journalctl _PID=XXXXX
Also, what caused/led you to do the following? Perhaps try undoing those changes as I'm not sure if that's helping or hurting. Was the /etc/init/isc-dhcp-server.conf file already there or did you manually create it?
So add a "Sleep 30" to the /etc/init/isc-dhcp-server.conf file. Also add " up service dhcp3-server restart " to my /etc/network/interfaces file.