It’s completely normal not to know the right answer to everything. In fact, we would rather you tell the customer that you don’t know the correct answer but are happy to help, rather than suggest the wrong product!
When facing this situation, the best practice is to keep the customer engaged by letting them know that you are here for them and it will take you a moment to look this information up. Online customers are human and as long as you remain polite and professional, they will be willing to hang around and wait for your reply!
If you absolutely don’t know the answer to their question and you’ve exhausted all options (expert resources, training resources, sent a message to the community of fellow experts), then let the customer know that while you cannot provide them with accurate information at the moment, you are happy to help them with other products. Let them know that you have addressed their question in the community and they should try coming back at a later time! Remember, you can always transfer a conversation to another expert, if needed.