First, the current supported VST3 SDK is 3.6.14 which I am assuming you have. It also looks like you have created a new project here - have you tried compiling the sample code first? I always compile the samples across Win and Mac for all APIs just before releasing any new SDK so you might want to always start with a sample first. As for your errors:

1. the last warning about EXPAT is from VST3 and you can ignore it. 

2. the second error is the VST Validator failing 

3. the first error is causing the Validator to not compile, which is producing the second error.

This is what I did to test the ASPiK SDK 1.6.4 sample project DemoVolumePlugin with VST3 SDK 3.6.14 with a note at the end about your error #1. 

I am using Win10 and VS 2019 16.4.2

After setting up the VST3 SDK according to the ASPiK docs, creating the myprojects subfolder and placing the vstgui4 subfolder within that. Then:

1. copied the DemoVolumePlugin into the myprojects folder

2. edit the CMakeLists.txt file to setup for VST3 and NON universal build

# --- Universal Build Flag - when using combined SDKs; can still build independently with individual flags below
set(UNIVERSAL_SDK_BUILD FALSE) # <-- set TRUE or FALSE # --- Individual project builds

set(AAX_SDK_BUILD FALSE)# <-- set TRUE or FALSE

set(AU_SDK_BUILD FALSE)# <-- set TRUE or FALSE

set(VST_SDK_BUILD TRUE)# <-- set TRUE or FALSE

3. Ran CMake:

cmake -G"Visual Studio 16 2019" ../

You can ignore the messaage about EXPAT (this comes from VST3, NOT ASPiK)

The success of CMake is indicated here:

Build files have been written to: C:/ALL_SDK/VST_SDK/VST3_SDK/myprojects/DemoVolumePlugin/win_build

4. open win_build/DemoVolumePlugin.sln with Visual Studio 2019 16.4.2

5. Build -> Rebuild ALL_BUILD

My build was successful, and the validator ran, and the file was written into the vst3 plugin bundle correctly. Here is what it looks like in VS 2019 v16

So, the sample code compiles correctly and the plugin is created. 

Now as for your first error, I have never seen this before. I did a google search and found that it was from the include shown in your error. I searched the VST3 library, and found the #include inside of the ..\VST_SDK\VST3_SDK\public.sdk\source\vst\hosting\module_win32.cpp

This file is used in the validator application. I found the offending code at line 44 in the module_win32.cpp file, BUT notice the #define before it:

// The <experimental/filesystem> header is deprecated. It is superseded by the C++17 <filesystem> header.

// You can define _SILENCE_EXPERIMENTAL_FILESYSTEM_DEPRECATION_WARNING to silence the warning,

// otherwise the build will fail in VS2019 16.3.0

#define _SILENCE_EXPERIMENTAL_FILESYSTEM_DEPRECATION_WARNING

#include <experimental/filesystem>

------------

The #define _SILENCE_EXPERIMENTAL_FILESYSTEM_DEPRECATION_WARNING eliminates the error you are seeing.

When I removed this #define, my Validator compile failed with the same error as yours. But, this line of code should not be removed, and arrives intact within the VST3 SDK 3.6.14 product. 

Are you using the correct VST3 SDK?

Will