One of my F# projects started throwing this error on build after I uninstalled and reinstalled some other, unrelated .net core versions.

    C:\Program Files\dotnet\sdk\5.0.103\FSharp\Microsoft.FSharp.Targets(281,9): error MSB6003: The specified task executable "fsc.exe" could not be run. System.ComponentModel.Win32Exception (193): The specified executable is not a valid application for this OS platform. [C:\FSharp.fsproj]
    C:\Program Files\dotnet\sdk\5.0.103\FSharp\Microsoft.FSharp.Targets(281,9): error MSB6003:    at System.Diagnostics.Process.StartWithCreateProcess(ProcessStartInfo startInfo) [C:\FSharp.fsproj]
    C:\Program Files\dotnet\sdk\5.0.103\FSharp\Microsoft.FSharp.Targets(281,9): error MSB6003:    at System.Diagnostics.Process.Start() [C:\FSharp.fsproj]
    C:\Program Files\dotnet\sdk\5.0.103\FSharp\Microsoft.FSharp.Targets(281,9): error MSB6003:    at Microsoft.Build.Utilities.ToolTask.ExecuteTool(String pathToTool, String responseFileCommands, String commandLineCommands) [C:\FSharp.fsproj]
    C:\Program Files\dotnet\sdk\5.0.103\FSharp\Microsoft.FSharp.Targets(281,9): error MSB6003:    at Microsoft.Build.Utilities.ToolTask.Execute() [C:\FSharp.fsproj]

Googling the error didn’t turn up anything useful. I started a new F# project which compiled without errors, which led me to the solution, which was to delete my failing projects .idea folder. After reopening Rider, problem solved.