(Legally obligatory notice - I work for Dell)
Checking the GPU Riser Config
When I was configuring the server on Dell's website, there was an
option to include an A40. I included it so that their system
configuration checker would give an error if the server wouldn't be
compatible with the A40. When the checker wasn't giving any errors, I
then removed the A40 and took that configuration.
I don't use the public site, but it sounds like you already double checked that you do have the GPU risers. Looking through the technical guide it looks like the board is the same regardless, it's just the PCIe risers that change. I'm not sure if they have this option on the public site but in our sales tools we have to pick the GPU ready option to get a chassis config'd for a GPU without actually shipping it with the GPU itself. I'm not sure that it would work just selecting and unselecting the A40 in a T550. I'm pretty confident that wouldn't work for an R7525 for example. The option looks like this for me on the R7525:
I don't see that option for the T550 which makes me think you either get the server with a GPU or you don't but I'm not familiar with the tower servers. If you have the GPU configuration the risers should look like this:
GPU Cable
There are 2x 8-pin sockets on the T550's power board labelled GPU_PWR1
and GPU_PWR2. What type of sockets are these, and what are they for ?
They are smaller and have a different pin shape compared to an 8-pin
CPU socket. Note: The 4-pin socket labelled GPU_PWR3 (down and left of
the 8-pin sockets) supplies the GPU risers.
It's this cable I think - part number FWCHY description ASSY,CBL,PWR,2X4,GPU,TWR
. If the tower servers work like the datacenter servers do this indicates that the server was not bought with the GPU config. At least in something like the R7525, in the GPU ready configuration, it will come with the correct cables in place. That said, I am not familiar with the T550.
Getting the Cable
This is a bit trickier if you aren't a large customer with a dedicated sales team. When I've had to deal with this in the past we have to go into a special tool and then seed just that esoteric part. It's a pain and that's why the GPU-ready config exists - the system is not set up to sell just that random part. I would call the small business line, tell them what happened, and ask for that part number I listed. You can give them my name and point them to this post if they have questions. This isn't something they likely deal with often so they may be a bit confused. Edit: Or if you are a larger customer the sales team's inside sales rep will know how to deal with it.
Favor: if this is helpful to you mark it as the answer so that I can show the powers that be that I'm being useful on here . (A lot of new people will just leave it in the void)