I tried to send a few ships to a galaxy a couple of systems away from mine to recycle a debris field and to see how fleet saving would work out. (Found out the hard way fleet saving is a good idea ) I went to "fleet", selected the ships I wanted to go, and put in the coordinates. But then I keep getting "mission invalid" and I can't figure out what I'm doing wrong. I had enough deuterium to send them. Any help is appreciated.

The target debris field should have planet on it.
Do not input more resources than you have. The mission consumes fuel so don't input the current deuterium. Better use max.