Very few things separate generations like meme language. Most under 35s will recognize this phrase immediately, and will be able to picture the meme it originally came from. People who are a little older will wonder why this gaming store uses such bad spelling and grammar.
For those not in the know, 'Y U No' translates to 'why haven't you?' This customer is being asked why they haven't pre-ordered a brand-new video game - although that's really none of the store's business.