Cant equip item in off-hand without unequipping the item in my main hand

Can anyone help fix this problem? I’m a level 22 Worgen Priest and whenever I try to equip an off-hand item while having an item in my main hand the off-hand item gets equipped but the main hand item is unequipped

The item in your main hand is probably a 2-handed weapon (like a Staff)

As a priest you can either use a 2-handed Staff which basicly takes up both the weapon slots (even though that isn’t obvious from the paperdoll interface),
or you can use a Mace, Dagger or Wand in your main hand which allow you to equip an off-hand as well.