Which one is not working when you check the rear lamp cluster? It is suggesting the Left tail light bulb. If it is this then it is usually a 5W single filament.
There are 3 types of bulb in the tail light cluster on the W209. The Halfords GE bulb numbers are 207 - 12V 5W R5W (the smallest one in the cluster), 380 - 12V 21/5W P21/5W - the twin filament bulb, there's only one of those - the rest are 382 - 12V 21W P21W. Last time I got that message I got a pack of each - always handy to have spares. Not sure which one it was but I have one of the 207's and one of the 382's missing from my packs.
And just to confuse you even more, if at all possible it will do the same on the other side to make the lights still appear symetrical., So, looking at the lights they all appear to be OK.
They will look ok because as said the car will use alternative illumination so it appears normal. Buy Osram or Bosch brands, Ring are quite poor quality.