Cannot resolve symbol 'app'

This error typically occurs when there is an issue with the Gradle build files or the AndroidManifest.xml file in an Android project.

To resolve this error, try the following steps:

1. Make sure that your Gradle build files are synced and up-to-date. You can do this by clicking on the "Sync Now" button in the Gradle Console or by selecting "File" > "Sync Project with Gradle Files" from the menu bar.

2. Check that your AndroidManifest.xml file is correctly formatted and does not contain any errors. You can do this by opening the file and checking for any red underlines or error messages in the code.

3. In cases where you are working with multiple modules or libraries, make sure that you have correctly defined the dependencies in your build.gradle files.

4. If you are still unable to resolve the issue, try cleaning your project by selecting "Build" > "Clean Project" from the menu bar, and then rebuilding it by selecting "Build" > "Rebuild Project".

If none of these steps work, you may need to do further troubleshooting or seek assistance from the Android developer community.