Okay, maybe I will just list some rules with regards to equipping weapons and see whether any of them applies to you.
- When you have a weapon in your off-hand, you may not equip a two-handed weapon in your main-hand.
- You may not dualwield a ranged weapon and a melee weapon at the same time, even if the ranged weapon has melee capabilities.
- You may not equip two launchers (bows and crossbows for example) of the same type at any one time.
- If you have Melf's Minute Meteors casted, or you have shapeshifted, you may not equip or remove anything from your weapon slots.
There are more, but I think these are the more relevant ones. In case this still does not help, what is your Bard kit, and what weapon are you trying to equip on him/her?