So namida just posted a bug involving pickup skills that triggers an error message. I'm getting a similar sort of thing here which according to the level doesn't have any pickup skills at all, but instead had several updrafts, and pre-assigned gliders.
Basically, the error message pops up when loading this level in the editor, but is still playable in the player. It can be found the "War" rank of the "Lemmings United".
I've already fixed an issue that's the likely cause of this, just haven't released the update yet. I should get that done.
Please try the new editor update and let me know if the error still occurs. If it does, please post a screenshot of the error message.
Just got the editor update and now the level's working. Thanks, namida. 8-)