In Hero Lab, it's only an error message if you actually buy the amulet - it still functions, and it doesn't prevent you from making the purchase, so I'd just buy it for free and ignore the error message, then pay the correct cost for the enchantment - that's if your GM allows this to happen, since strict RAW seems to suggest that monks in ABP get penalized by not having access to enchantments like other characters would, but that seems like a reasonable exception that I'd expect GMs to allow.