There is no bifurcation settings in BIOS.  There are other related settings.

Default settings for miniSAS ports, all drives are enables.

Default settings for Intel VMD (RAID), all front PCIe bay are enabled.  All PCIe slots are on Auto mode (enabled).

Have you test the Seagate card on other x16 slots connected to 1st CPU to verify its working condition.

• slot n.6 is related to the 2nd cpu and is located at the top of the case. What number of slots and for the 1 cpu.

Starting with the closest to CPU, test with slot #2 and slot #4 as they are wired for x16 lane.

Edited:  Just note that DELL-Cares referred you to bus allocation.  Those options are for thunderbolt use.  But the default settings should already allocated enough PCIe lane for 2nd CPU slots.  

I apologize for the long wait today. 

Here is a link you may refer to for Pcie bifurcation process.

https://dell.to/3OkqT0U

Please ensure you do not do this in the drive that has the OS installed on it, to avoid data loss.

How should I deal with the settings in this section of the bios shown in the link?
The board is positioned in slot n.6 cpu 1.
I attach image.

Thanks for the attention.

In the bios the settings are like in your post mentioned before.
I tried the Seagate card in other slots as well to no avail.
Even a third-party (non-dell certified) card from Kalea Informatique won't work.
I remember that on the board there are 4 NVMe M.2 SSDs of 2TB each 970 EVO Plus Samsung
to partition and format.
I am also attaching photos of the bios information the card is allocated to slot 6 cpu 1.

Thanks for the attention.BIOS 7920 Tower

I see that you already have a video card installed on slot 2 and a thunderbolt card installed on slot 4. 

There are 2 tests to perform, first, remove the thunderbolt card, and installing the Seagate card on slot 4.  This will test the functionality of the Seagate card.  Second, remove the video card from slot 2 and installing it to slot 6_CPU1, this will test the functionality of PCIe slots from 2nd CPU.

Note that your machine will restart 3 times before boot up, this is normal as it makes adjustment when seeing hardware changes.

Also note that an empty adapter card won't show up in BIOS, it must have nvme drive(s) installed.

After having test results, you can restore the video card back to slot 2 and thunderbolt card back to slot 4.

I installed the graphics card in slot 6 cpu1 and the Seagate in slot 4.
Removed the Thunderbolt.
The machine boots up and the graphics card works but the Seagate is not detected.
I was wondering but the disks installed on the Seagate are still to be initialized and partitioned and then formatted, is it possible that it is not detected for this?

Nah, something is wrong with the Seagate card.  Just get yourself a generic adapter such as Asus Hyper and transferring all NVMe drives over.

You can put the video card and thunderbolt card back to their original location, and closing this thread as nothing is wrong with your workstation or the BIOS settings.