Small bug, possibly long-standing, probably not noticed by most long-term players with a massive load of perks. Let's imagine a situation where you have enough perks for a single B-29, but not enough for a formation. You've set the B-29 to take off solo.
You're diddling around with a formation of Lancasters in the hangar, pondering different bomb loads. Then you click to select your solo B-29. "Not enough perks for that model", even though you have more than enough for a single bomber.
What seems to happen is that the "formation" flag on the clipboard is toggled after the game checks for the perk cost of the unit, rather than before.