no main manifest attribute, in /app.

It seems like you are encountering a "no main manifest attribute" error. This error occurs when the JAR file you are trying to execute does not have a specified main class in its manifest file.

To fix this issue, you need to make sure that your JAR file includes a manifest file with the correct main class specified. The manifest file should be named "MANIFEST.MF" and placed in the "META-INF" directory of your JAR file.

The content of the manifest file should look like this:

Manifest-Version: 1.0
Main-Class: your.package.name.YourMainClass

Replace "your.package.name" with the actual package name of your main class and "YourMainClass" with the name of your main class.

Once you have created or updated the manifest file, repackage your application into a JAR file and try running it again. The error should be resolved.

If you are using a build tool like Maven or Gradle, they usually handle the creation of the manifest file for you based on your project configuration. Make sure to check your build tool's documentation for more details on how to configure the main class in the manifest file.