From your image and what I can also find on the web this is confusing! At is implies there is the option of adding additional RAM.
From their website: “DDR4 2133MHz SDRAM, 1 x SO-DIMM socket for expansion, up to 16 GB SDRAM”
Clearly this model has 8 GB of soldered of RAM (DDR4 2133MHz SDRAM) which is likely located under the shield given the limited view of your image. There should be a DIMM slot nearby (1 x SO-DIMM socket for expansion). Based on the description, but I don’t see it. If you peel back the tape does that offer a clue?
Looking at this repair guide which I found: Service Manual - ASUS Zenbook UX501VW. There appears to be two versions of the logic board! Your image does not show the signs of a socket where this manual shows one.
‘’’So... What to do to fix this?’’’ First I think you’ll need to talk with ASUS on getting clearification on what the web site shows (and hopefully your users guide as well) and the fact the model they sold you does not offer the option to expand the RAM. You’ll need to see what they will do to rectify this either giving you your money back of offer exchanging it for the 16GB model.
If that fails you’ll need to talk with your local consumer protection agency or if that’s not an option speak to a lawyer. Make sure you have all of your paperwork of your purchase and take notes of all your calls and what was stated (record them if you can) as that may help you later.