Dang, while it may not help much, here is a picture of the affected area:

I'd guess it's around 10uF based on the size. If you can remove and measure C13 or C12, that'd be most accurate, as they look like the same one.

I tried to measure it in circuit - didn't work, the measurement was jumping between uF and nF scales back and forth without indicating any value.

For those tiny capacitors, when values are not known, I generally just pull one the same size and color off a parts board.

As long as the value is the same or greater you should be fine.

its also just a bulk power decoupling cap, judging on the traces/placement/etc.

grab one the same size off a scrap board and you'll be fine.

Just to add to what others have already said, I've done exactly what they all suggest countless times.

I've pulled SMD caps from boards to measure out of the circuit. It's super easy to do with a soldering iron and just about any relativity modern digital multimeter.
https://www.wikihow.com/Measure-Capacitance

However, in most cases like yours, I just grab one that physically looks the same (color and size) from a donor board and drop it in. Over 30 years repairing electronics and never had an issue with this method.

On a side note, I have the same card, so I took a look just to verify mine are not marked either.

C11...13 should be something around 22...47pF range, they will be difficult to measure even out of circuit due to the small capacitance.
EDIT : They are all 56pF, I just measured them on a card I have.

They're all sitting between ground and #dack5/#dack6/#dack7, filtering some spikes in the lines.
It just might work as is without caps, or at least the DMA 6 should be avoided if the missing cap is of any concern.

Yeah, they are there for glitch filtering and in all likelyhood the board works fine without them.