First, you need to verify which version of Java you have installed on your system.If you're indeed using Java 11.0, you should install an earlier version of Java that is compatible with the Android Designer. Historically, Android development has been most compatible with Java 8.
After installing the compatible version of Java, you may need to configure your system to use that version. This might involve updating the JAVA_HOME environment variable to point to the directory where the compatible Java version is installed.
Make sure that your Visual Studio for Mac is up to date. Sometimes, newer updates include fixes for compatibility issues.
Check your Xamarin.Android project settings within Visual Studio for Mac to ensure that it's using the correct Java version. You might need to specify the path to the compatible Java SDK in your project settings.
If there are any project files or configuration files where you can specify the Java SDK version, review them to ensure that the correct version is being used.