/platform:[platform]
This option, input as a string, is mandatory for publishing test results. The values of /flavor and /platform must match a valid combination of values for the completed build identified by the /publishbuild option. When you run the MSTest utility with the options for publishing, it first finishes running tests and then it sets the flavor and platform properties of the test run before publishing the test run results.
Examples of using /platform: /platform:AnyCPU, /platform:x86