Make sure that you have an IP Pool assigned to the relevant DHCP server. Post output from /ip dhcp-server for comment.
0 name="Primary DHCP Server" interface=ether1 lease-time=2h address-pool=primary_pool bootp-support=static authoritative=after-2sec-delay lease-script=""
1 name="Secondary DHCP server" interface=ether1 relay=255.255.255.255 lease-time=2h address-pool=secondary_pool bootp-support=static authoritative=after-2sec-delay
and my pools:
0 primary_pool 192.168.1.100-192.168.1.254
1 secondary_pool 192.168.2.2-192.168.2.254
in primary DHCP Server address-pool was static. (now changed to primary_pool because with static it's doesn't work)