I don't recall getting an invalid target message. Could that be that you're getting no free slots message instead? Every system body has a limited amount of slots for carriers, so the system can only hold as much carriers, as many slots all its bodies have. Currently Upaniklis is (surprisingly) full and there're no slots in it.
As an added note, you're getting the same message if you're trying to jump to an orbital starport or station "orbit", clicking on said starport. Yeah, they don't have their own orbit, hence no slots there, so the game reports you that. Usually red carrier icons by the system bodies indicate that their slots are full, you can spot em on sys map.
So, you can just test it. Pick any station in any system and try to target it for the jump. Is this that behaviour that you're talking about, or that's something else? Can you jump for just some other random planet then (jumps can be cancelled after scheduling for at least 5 mins, so you can cancel one even if scheduled). Or maybe in your case the game is throwing random messages at you while the reason is still just Upaniklis is full.
Personally i find jumping in a (currently) popular system isn't a best solution. It's cluttered with a lot of carriers, and nothing guarantees you from being directed to the 1st out of 7 moons gas giant to crawl your way between them and then go do a 2k ls supercruise. Better solution for me is to park in a nearby system, preferably with just one single star, at that star, and then just jump where you need. Like you're going to Shinrarta Dezhra, look at LHT 926 traffic. 300+ ships. A system Nortia that is nearby has a traffic of less than 5 ships, has a guaranteed spot at the star for your carrier (no planets there) and i never seen it full. But ppl go for LHT 926 obviously, it's closer! Even if you can't jump 20 or so ly (unlickely) between em, 2nd jump is much faster than supercruise on your return.
Ofc there can be a need to park at a planet when you need to get to some very far from main star one, or to mine for example, but when travelling i find it better to be at the star in nearly all cases.
Otherwise ye, FC faq is good and there's a lot of q answered in the following posts too, worth looking through all the pages, and guess better to keep Q@A there. I gathered there that you can't actually trade rares from your carrier, i never knew.
