I had a similar problem and it turned out that during a merge, in my csproj file, the PropertyGroup (labelled SlowCheetah) went down the Import element.
It should be like this
<PropertyGroup Label="SlowCheetah">
<SlowCheetahToolsPath>$([System.IO.Path]::GetFullPath( $(MSBuildProjectDirectory)\..\packages\SlowCheetah.2.5.10.3\tools\))</SlowCheetahToolsPath>
<SlowCheetah_EnableImportFromNuGet Condition=" '$(SC_EnableImportFromNuGet)'=='' ">true</SlowCheetah_EnableImportFromNuGet>
<SlowCheetah_NuGetImportPath Condition=" '$(SlowCheetah_NuGetImportPath)'=='' ">$([System.IO.Path]::GetFullPath( $(MSBuildProjectDirectory)\Properties\SlowCheetah\SlowCheetah.Transforms.targets ))</SlowCheetah_NuGetImportPath>
<SlowCheetahTargets Condition=" '$(SlowCheetah_EnableImportFromNuGet)'=='true' and Exists('$(SlowCheetah_NuGetImportPath)') ">$(SlowCheetah_NuGetImportPath)</SlowCheetahTargets>
</PropertyGroup>
Then
<Import Project="$(SlowCheetahTargets)" Condition="Exists('$(SlowCheetahTargets)')" Label="SlowCheetah" />
Here is why the order of elements is important in a build file. Hope it helps