I, too, encountered this problem while trying to build a plugin using Visual Studio 2017 on my horribly messed-up workplace computer. If you search the internet for "unable to find resgen.exe," you can find all this advice that's like 'just use regedit to edit your Windows Registry and make a new key here and copy-and-paste the contents of this folder into this other folder, blah blah blah.'
I spent weeks just messing up my Windows Registry with regedit, probably added a dozen sub-keys and copy-pasted ResGen.exe into many different directories, sometimes putting it in a 'bin' folder, sometimes just keeping it in the main folder, etc.
In the end, I realized, "Hey, if Visual Studio gave a more detailed error message, none of this would be a problem." So, in order to get more details on the error, I ran MSBuild.exe directly on my *.csproj file from the command line:
"C:/Windows/Microsoft.NET/Framework/v4.0.3.0319/MSBuild.exe C:/Users/Todd/Plugin.csproj -fl -flp:logfile="C:/Users/Todd/Desktop/error_log.log";verbosity=diagnostic"
Of course, you'll have to change the path details to fit your situation, but be sure to put 1) the complete path to MSBuild.exe 2) the complete path to your *.csproj file 3) the -fl -flp:logfile= part, which will tell MSBuild to create a log file of each step it took in the process, 4) the location you would like the *.log file to be saved and 5) ;verbosity=diagnostic, which basically just tells MSBuild to include TONS of details in the *.log file.
After you do this, the build will fail as always, but you will be left with a *.log file showing exactly where MSBuild looked for your ResGen.exe file. In my case, near the bottom of the *.log file, I found:
Compiling plug-in resources (Task ID:41)
Looking in key SOFTWARE\WOW6432Node\Microsoft\Microsoft SDKs\NETFXSDK\4.6.2\WinSDK-NetFx40Tools-x86 (Task ID:41)
Looking in key SOFTWARE\WOW6432Node\Microsoft\Microsoft SDKs\NETFXSDK\4.6.1\WinSDK-NetFx40Tools-x86 (Task ID:41)
Looking in key SOFTWARE\WOW6432Node\Microsoft\Microsoft SDKs\NETFXSDK\4.6\WinSDK-NetFx40Tools-x86 (Task ID:41)
Looking in key SOFTWARE\WOW6432Node\Microsoft\Microsoft SDKs\Windows\v8.1a\WinSDK-NetFx40Tools-x86 (Task ID:41)
Looking in key SOFTWARE\WOW6432Node\Microsoft\Microsoft SDKs\Windows\v8.0a\WinSDK-NetFx40Tools-x86 (Task ID:41)
MSBUILD: error : Failed to locate ResGen.exe and unable to compile plug-in resource file "C:/Users/Todd/PluginResources.resx"
So basically, MSBuild looked in five separate directories for ResGen.exe, then gave up. This is the kind of detail you just can't get from the Visual Studio error message, and it solves the problem: simply use regedit to create a key for any one of those five locations, and put the value "InstallationFolder" in the key, which should point to the folder in which your ResGen.exe resides (in my case it was "C:\Program Files\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.7.2 Tools").
If you're a humanities major like myself with no background in computers, you may be tempted to just edit the heck out of your Windows Registry and copy-paste ResGen.exe all over the place when faced with an error like this (which is of course, bad practice). It's better to follow the procedure outlined above: 1) Run MSBuild.exe directly on your *.csproj file to find out the exact location MSBuild is looking for ResGen.exe then 2) edit your Windows Registry precisely so that MSBuild can find ResGen.exe.
