@dvkch Thank you for reporting this issue.
I also reproduced the issue.

I found that it is possible to build using the workaround below, but it does not solve the fundamental problem.

@main
struct PrepareLicenseList: BuildToolPlugin {
    // ...

    // This command works with the plugin specified in `Package.swift`.
    func createBuildCommands(context: PluginContext, target: Target) async throws -> [Command] {
+       var executablePath = try context.tool(named: "spp").path
+           .removingLastComponent()
+           .removingLastComponent()
+           .appending(["Debug-maccatalyst", "spp"])
        return [
            makeBuildCommand(
+               executablePath: executablePath,
                sourcePackagesPath: try sourcePackages(context.pluginWorkDirectory),
                outputPath: context.pluginWorkDirectory
            )
        ]
    }
}

https://github.com/cybozu/LicenseList/blob/support-catalyst/Plugins/PrepareLicenseList/main.swift

#if targetEnvironment(macCatalyst) preprocessor also doesn't work. X(

Oh! I didn't know you could modify the build commands like this, that's good to know! But as you say, the underlying issue is still there

I will try this problem again.

@dvkch
I tried it and it worked. #50

This seems great ! Do you think there would be a way to detect if we are building Release vs Debug ? Maybe something like this instead ? But honestly even if the env doesn't match, it seems already really great to have spp work for Catalyst !

   let macCatalystPath = try context.tool(named: "spp").path.string
      .replacingOccurrences(of: "/Debug/", with: "/Debug-maccatalyst/")
      .replacingOccurrences(of: "/Release/", with: "/Release-maccatalyst/")

@dvkch I forgot about release builds.
Is the Debug-maccatalyst directory created even if you only do release builds?

And I realized that there is a problem with workaround.
#50 (comment)

I don't think it is, but it might still exist from a previous Debug build?