'Without memory' with memory...

Hello friends.

I am just finishing my first PC/ server build and when I boot up I get 3 beeps which, I believe, is telling me I am 'without memory' - see AMI UEFI BIOS codes here:

https://www.asrock.com/support/QA/TSDQA-84.pdf

I have a ASRock - E3C222D4U Micro ATX LGA1150 Motherboard which supports Dual Channel DDR3 1600/1333 ECC DIMM, and has 4 x DIMM slots:

https://www.asrockrack.com/#Specifications

I have 4 x 4gb DDR3 PC3 10600R 1333 240Pin RDIMM ECC Registered RAM sticks from here:

https://www.ebay.co.uk/itm/Memory-Ram-4-Server-DDR3-PC3-10600R-1333-240Pin-RDIMM-ECC-Registered-2X-GB-Lot/322702953843?hash=item4b22984d73%3Am%3AmAxt4ZhHVXkmlRDdqqMGDkQ

I have tried using just one stick in each slot and no luck - am I missing something obvious?

If I had any hair I'd be pulling it out right now. Grateful for any help from the community.

Cheers

Lemon