I reinstalled Windows 10 last night due to an issue after the recent Windows 10 version 2004 patch.  That issue was resolved with a reinstall.  I had SLI working in this version of Windows 10 and earlier versions of WIndows 10 without an issue before the reinstall.  Now I can't enable SLI through the Nvidia control panel.  When I click on "Maximize 3D Performance" then click on apply Windows make the found new hardware sound, the screen goes black then it makes the sound you hear when you unplug a USB device and the screen image comes back and shows that SLI is not enabled according to the Nvidia control panel. Also, I close the programs that need to be closed according to Nvidia control panel and it allows me to proceed.
 
I've tried several recent driver versions with using DDU in between them and none of them worked to enable SLI.  I've tried taking the card out and putting them back in then connecting the NVLink bridge and that hasn't worked.  I've tried booting without the NVLink bridge installed and installing the drivers, then shutting down the PC and installing the NVLink bridge and rebooting.  I get the message about the system is capable of higher performance and when I go to enable SLI, I get the same issue where it doesn't work and windows makes the same sounds.
 
How do I get this to work? Is there some sort of log or diagnostic file I could look at?

That didn't work, but thanks for the suggestion.  I still have the same issue.
 
This is a new windows install with next to nothing on it other than Windows 10 Pro and Office 2019.

Both cards show up in device manager without errors.
 
I've reseated both cards and the SLI bridge 3 times already without it making a difference.
 
I've also put in new power cables recently because the ones that came with my EVGA power supply were defective.  I checked and the connections are all secure.
 
I'm guessing it might be a windows issue, because SLI worked fine before the reinstall.

I just did a new install of Windows 10 and with nothing else installed.  I rebooted then tried enabling SLI.  I couldn't enable SLI.  It did the same thing as before.  When I click on "Maximize 3D Performance" then click on apply Windows make the found new hardware sound, the screen goes black then it makes the sound you hear when you unplug a USB device and the screen image comes back and shows that SLI is not enabled according to the Nvidia control panel.  I tried both the latest drivers and version 451.67 with using DDU in between while in safe mode to uninstall them.  Neither worked.

bob16314
First of all, grab GPU-Z and see what the "Bus Interface" is..It must say either "PCIe x16" or "PCIe x8" for SLI to work/be enabled.

That was one of the first things I checked.

Check all of your power supply connections to the cards as well and see if one is not fully seated. What motherboard are you using, make sure on of your nvme drives is not in a slot that taking pcie lanes from your gpu slot being used. That x8 1.1 mode seems troubling as well, did you reseat the cpu ? Did both gpu's show the same thing in gpuZ ? Did you download and install the latest chipset drivers for you motherboard as well? 

sparetimepc
Check all of your power supply connections to the cards as well and see if one is not fully seated. What motherboard are you using, make sure on of your nvme drives is not in a slot that taking pcie lanes from your gpu slot being used. That x8 1.1 mode seems troubling as well, did you reseat the cpu ? Did both gpu's show the same thing in gpuZ ? Did you download and install the latest chipset drivers for you motherboard as well? 

Its the MSI MEG x570 ACE motherboard.  I had SLI working for 6 months with he exact same hardware in this motherboard before I reinstalled Windows recently.  So its not a hardware limitation due to a NVMe drive taking up PCIE lanes.  Why would the x8 mode seem troubling?  Its completely normal for the slot to be in that mode when you have two GPUs installed.  I downloaded and installed all the drivers for the motherboard chip set and all the other hardware.  

sparetimepc

bavor

sparetimepc
Check all of your power supply connections to the cards as well and see if one is not fully seated. What motherboard are you using, make sure on of your nvme drives is not in a slot that taking pcie lanes from your gpu slot being used. That x8 1.1 mode seems troubling as well, did you reseat the cpu ? Did both gpu's show the same thing in gpuZ ? Did you download and install the latest chipset drivers for you motherboard as well? 

Its the MSI MEG x570 ACE motherboard.  I had SLI working for 6 months with he exact same hardware in this motherboard before I reinstalled Windows recently.  So its not a hardware limitation due to a NVMe drive taking up PCIE lanes.  Why would the x8 mode seem troubling?  Its completely normal for the slot to be in that mode when you have two GPUs installed.  I downloaded and installed all the drivers for the motherboard chip set and all the other hardware.  

Not the x8 so much its the 1.1   

Probably the power saving mode - unless that is what shows when he puts it under "test" or other 3D load.

So are you saying when you go into device manager, there are no errors in the whole list when the gpu's and the nvlink bridge are installed but still when you try and enable SLI it won't do it? 

Ya did install the chipset drivers right?

I have the latest version of windows.  That tool says that I already have the latest version of Windows 10.
 
Here is a screenshot of the OS info from my about your PC page

I don't have that option because SLI is not enabled.  That option only shows up after you have SLI enabled and I can't enable SLI in the first place.

The monitor is hooked to the top GPU with a DVI cable.
 
When I remove the bridge, I don't have the option to enable SLI appear in Nvidia control panel.

Both GPUs say PCI Express x8 Gen3 there.  Both have the same BIOS revision also.

I think I would reinstall Windows/Drivers/Updates with just one card installed, then add the second one when you're all done..Did you try that?