Have you been able to put the same character into a combat slot before?  If not, make sure they have the "combatant" training.  If so, are they injured from a previous battle?  (There should be an icon in their status if so.)  In that case, they won't be able to fight until their injury heals.  

The equipped weapon matters for damage/reach, but shouldn't matter for whether or not you can place a character into a combat slot at all.

Please provide a save file when it happens if you don't use mods