The error you are getting is due to the VW Canbus wiring system. Essentially, the LED's are such a low voltage that the car thinks that the bulb has blown.
So you need to buy proper Canbus compatible LED's. Most of the ones on ebay are rubbish though. I have a set of the lights linked below and they are excellent - brighter than stock with a nice crisp white light, without being super super bright like some of the replacement lights can be.
VW Scirocco Polo Passat Golf MK4 MK5 MK6 GTI LED License Plate Lights | eBay
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