The error message “MainActivity.java is not on the classpath of project app, only syntax errors are reported” typically indicates an issue with the Java Language Server not recognizing the MainActivity.java file as part of the classpath in your Android Studio project. This can lead to the IDE only reporting syntax errors without properly recognizing the structure of your project.

Solution:

Press “Ctrl + Shift + P” (Windows/Linux) or “Cmd + Shift + P” (Mac) to open the Command Palette in Android Studio.
type “java : Clean Java Language Server workspace” and select this option.
Press Enter to run the selected command. This action will clean the Java Language Server workspace, which can help resolve issues related to classpath recognition.
If the problem persists, try the following additional steps:

a. Right-click on the src folder in your project.

b. Choose “Add Folder to Java Source Path.” This action explicitly adds the src folder to the Java source path, ensuring that the Java Language Server recognizes it. restart Android Studio to apply the changes.