I've seen this same problem.

I believe I've completed the single player storyline and I've returned to Manhattan.

My next level requirement at first said 113,000 credits worth. It slowly came down as I made money and it says my next level requirement is now 76,074 credits worth when I'm worth 103,034 credits.

So I can't level further or get into better ships than the Anubis VHF because I can't seem too level past 18.

File this under, "wtf?" because it seems like theres supposed to be more to the game...or possibly more to the SP story line (as I know I haven't even come close to visiting all the star systems I've been to in MP), but the game doesn't seem to want to let me advance further.

After the single player campaign is completed, the Player Info window will display the net worth you need to make from this point on to level up. So if it says you need to make $70K, then make $70K and you'll level up.

As for "Mission", I had that happen to me once, but I ignored it and at one point it ended up levelling up by itself.

I have gotten to level 28 in single player, left it for about a month, came back and it said needed a mission to level. I thought it would fix itself if I just got passed the level requirement but I'm now worth over 1 million and still haven't been able to level. On top of that, the NPC's are treating as if I'm a level 13 or 14 pilot so I can't get any mission above level 21