Namespace not specified. Specify a namespace in the module's build file

From the error message, it looks like you are trying to use Gradle 9.

Capacitor 3 uses Gradle 7 (source) so you probably need to downgrade.

In Android Studio go to:

• File → Project Structure… → Project

Then change Gradle Version to 7.0 and Android Gradle Plugin Version to 4.2.0.