From some time now I do use this bitfield for taking off (work with version 1.6.5) : (0x1000 | 0b100111000000)
For target.z you can try current.z - 5; (this will request a takeoff point 5 meters above the current location, as the z axis in inversed).
Are you in correct offboard mode. To do so you have to send a HEARTBEAT at 2Hz and a request to switch to offboard mode MAV_CMD_NAV_GUIDED_ENABLE.
And then you also need to arm the drone using MAV_CMD_COMPONENT_ARM_DISARM. These two command are 'long command' type.
Hope this will help you. I'll be glad to help you if it's not the case