Uninstalling older versions of Visual Studio and installing everything new solved the problem. 

I have similar problem. I have removed visual studio a few times and  install again following all steps on the axi doc. I allways get the same error. I can install all other plugins so I guess is not a visual studio problem.

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\DLSS\DLSS.Build.cs(23,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'DLSS'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\DLSSBlueprint\DLSSBlueprint.Build.cs(24,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'DLSSBlueprint'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\DLSSEditor\DLSSEditor.Build.cs(25,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'DLSSEditor'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\DLSSUtility\DLSSUtility.Build.cs(24,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'DLSSUtility'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\NGXD3D11RHI\NGXD3D11RHI.Build.cs(25,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'NGXD3D11RHI'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\NGXD3D12RHI\NGXD3D12RHI.Build.cs(23,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'NGXD3D12RHI'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\NGXRHI\NGXRHI.Build.cs(24,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'NGXRHI'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\NGXVulkanRHI\NGXVulkanRHI.Build.cs(25,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'NGXVulkanRHI'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\NGXVulkanRHIPreInit\NGXVulkanRHIPreInit.Build.cs(24,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'NGXVulkanRHIPreInit'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\ThirdParty\NGX\NGX.Build.cs(25,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'NGX'

LogAximmetry: Display: Plugin installer: ERROR: Unable to compile source files.

LogAximmetry: Error: Plugin installer: Error: DLSS could not be built. UnrealBuildTool exited with 6.

Using latest unreal for aximmetry and ue4.26.1 and latest visualstudio community 2019 Any help will be appreciated

For tryponiz:

It seems like you have two instances of the DLSS plugin inside c:\UnrealEngine\UE_4.26\Engine\Plugins.
If you followed our docs, one is at c:\UnrealEngine\UE_4.26\Engine\Plugins\Marketplace\DLSS   -   this is correct.
However your logs show that there is another one at c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS   -   this should not be here.
Please delete this folder and run Aximmetry / Install Plugins again in UE for Aximmertry.

I have all drivers and upgrades, new visual studio version, and still have Error 6..
Looking at whole log, it says many times when compiling:  c1xx: fatal error C1076.

It says there is a stack overflow.  Compilation stops on lines:

LogAximmetry: Display: Plugin installer:   [585/602] Module.Engine.32_of_49.cpp
LogAximmetry: Error: Plugin installer: Error: DLSS could not be built. UnrealBuildTool exited with 6.

I am not a programmer to deal with something like that. Had no idea how yto fix, and need DLSS.

For Krzysztof_studio144:

Based on this error log the most likely scenario is that you run out of memory. Can you please tell us how much do you have?

For working with unreal usually 64Gb is recommended, 32Gb is fine (although some operation can be slowed), 16Gb is not enough in a lot of cases (system will run out of virtual memory + extreme slowdown)

Also make sure that only the necessary programs runinng when you do the plugin installation.

Best regards

Hello,
I have 64 Gb Ram. I think in nearest days I will try to restore whole system from clean backup, then repeat process of installation and let's see what will happen. 
Have a good day :-)

Hi Krzysztof,

Did the clean install fix the DLSS plugin installation problem?

Warmest regards