MSBuild not compiling with both /p:Configuration=Release and -p:Configuration=Debug - asp.net

Question

We are attempting to install a clean build of the newest version of Kentico (12) on Azure, using Kudu, in DEBUG configuration. We traditionally accomplish this by setting SCM_BUILD_ARGS to -p:Configuration=Debug, and all is well. But for some reason in this newer version, the project fails to compile, and I see that this is the command being run:
"C:\Program Files (x86)\MSBuild\14.0\Bin\MSBuild.exe" "C:\Users\User\Documents\GitHub\Project\CMS\CMSApp.csproj" /nologo /verbosity:m /t:Build /t:pipelinePreDeployCopyAllFilesToOneFolder /p:_PackageTempDir="C:\local\Temp\8d6d315018f5ff9";AutoParameterizationWebConfigConnectionStrings=false;Configuration=Release;UseSharedCompilation=false /p:SolutionDir="C:\Users\User\Documents\GitHub\Project\.\\" -p:Configuration=Debug
Notice both Configuration=Release and Configuration=Debug. The compilation errors are things like Invalid expression term 'string' on the following line of code:
bool licenseOK = LicenseHelper.CheckLicenseLimitations(FeatureEnum.Ecommerce, out int skuCount, out int maxSKUCount);
This doesn't highlight at all in Visual Studio in Debug or Release mode. I am also able to reproduce locally (as shown in my paths above), and if I change the two configurations to both be the same (regardless of whether I choose Debug or Release), it compiles just fine.
What I know I can do is create my own custom deployment command script file as a work-around, but I guess I don't understand why the above MSBUILD command isn't working in the first place.
Any ideas as to why this would work great for all of my ASP.NET projects prior to this one? One thing I know is that Kentico recently included the Roslyn compiler, which I am unfortunately not that familiar with - could that be part of the issue?
UPDATE #1: It appears all of the compilation errors have to do with code that is only valid in C# 7.0. That could be a clue.

Answer 1

It all came down to the fact that the newer versions of Kentico had code that was only valid in C# 7, and required MSBuild 15 to compile. Here's how I was able to set up Kudu:
Add a file named .deployment in the root of my repo, containing only the contents below. This tells Kudu to use a custom deployment script.
Download the deploy.cmd file from my Azure App Service via FTP to the root of my repo, and update it per https://github.com/projectkudu/kudu/issues/2350#issuecomment-373817740 to use the MSBuild 15 path. Details below.
.deployment file contents
[config]
command = deploy.cmd
deploy.cmd updates
1 - Add -MSBuildPath "%MSBUILD_15_DIR%" to nuget call. Example:
nuget.exe restore "{your .sln file path}" -MSBuildPath "%MSBUILD_15_DIR%"
2 - Replace %MSBUILD_PATH% with %MSBUILD_15_DIR%\MSBuild.exe in both occurrences under step 2. Build to the temporary path
This was all that was needed in order to get the build to complete.

Answer 2

As Keith mentions, it was a compatibility issue with C# 7.0 that Kentico 12 uses. This SO thread mentions that you need to run MSBuild version 15 for C# 7.0 code. I ran a similar debug command for my local Kentico 12 project using MSBuild 15, and it succeeded.