9V dc barrel plug doesn't work on UNO

Here is the power supply I bought: https://www.amazon.co.uk/gp/product/B017YW2ATK/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

The arduino runs off the USB power fine.

The arduino doesn't respond to having the 9v 2a power supply? I tested it with a multimeter and it reads 9V.

Is the supply center positive or center negative? The arduino wants center positive. That page says "tip negative", while is a bit ambiguous but sounds like it could be center negative. Luckily the barrel jack has a diode to prevent damage in this case.

But in the photo the symbol on the case looks like centre positive to me

Well, you have the power supply, you have a volt meter... just measure it and know for sure!

anthonyHope:
But in the photo the symbol on the case looks like centre positive to me

But then the photo is a generic one and it certainly isn't of any 9V 2A supply. So it definitely can't be trusted. Measuring is the only way.

Steve

The description clearly says

Plug specification: 5.5 x 2.1 mm; Tip Negative (-) design