A V051 message indicates that the C# project, loaded in the analyzer, contains compilation errors. These usually include unknown data types, namespaces, and assemblies (dll files), and generally occur when you try to analyze a project that has dependent assemblies of nuget packages absent on the local machine, or third-party libraries absent among the projects of the current solution.
Despite this error, the analyzer will try to scan the part of the code that doesn't contain unknown types, but results of such analysis may be incomplete, as some of the messages may be lost. The reason is that most diagnostics can work properly only when the analyzer has complete information about all the data types contained in the source files to be analyzed, including the types implemented in third-party assemblies.
Even if rebuilding of dependency files is provided for in the build scenario of the project, the analyzer won't automatically rebuild the entire project. That's why we recommend that, before scanning it, you ensure that the project is fully compilable, including making sure that all the dependency assemblies (dll files) are present.
Sometimes the analyzer may mistakenly generate this message on a fully compilable project, with all the dependencies present. It may happen, for example, when the project uses a non-standard MSBuild scenario - say, csproj files are importing some additional props and target files. In this case, you can ignore the V051 message or turn it off in the analyzer settings.
If you wish to learn which compiler errors are causing the V051 error, start the analysis of your projects with the analyzer's cmd version, and add the '--logCompilerErrors' flag to its arguments (in a single line):
PVS-Studio_Cmd.exe –t MyProject.sln –p "Any CPU" –c "Debug" --logCompilerErrors