What went wrong:
Plugin [id: ‘com.android.application’, version: ‘9.0.0’, apply: false] was not found in any of the following sources:
Gradle Core Plugins (plugin is not in ‘org.gradle’ namespace)
Included Builds (No included builds contain this plugin)
Plugin Repositories (could not resolve plugin artifact ‘com.android.application:com.android.application.gradle.plugin:9.0.0’)
Searched in the following repositories:
Gradle Central Plugin Repository
Google
MavenRepo
Obviously, if there is actionable data, clues or information within any of the errors, fix those problems. Each error entry may have more information than can fit in the console log so select each error and read the expanded view below it. Check the Editor log as well.
If that gives you nothing worthwhile (look to Google! You’re never the first one!), then…
First, make a blank project with a single blank scene and prove that it builds successfully.
If the blank project does NOT build, go fix your Unity installation or your other tools, such as Android SDK, NDK, JDK, etc. It may even be necessary to change to a different version of Unity3D. It is generally best to stay with LTS versions of Unity3D.
Until you can build a blank project to the target platform, don’t fiddle with anything else.
Once you can build a blank project, now bisect the problem by bringing over parts of your current project and building it one subsystem at a time, perhaps stubbing things out that might trigger compiler errors.
Most often things that prevent building are misconfigured or incorrectly-installed or incompatible third-party libraries such as Firebase or AdMob or IronSource.
Once you identify the subsystem, go to the documentation for it and make sure you are doing it correctly.
It may also be helpful to work through a tutorial or two for whatever subsystem is making the build fail.
Android build not building:
Circa July 2022 here have been reports of Unity’s installer failing to install the Android Tools.
Here was how I brought up Unity2020.3.41 and the Android SDK 31 on October 30, 2022 (edited on August 23, 2023 to include Android SDK 33…) (and again for SDK34):
And for Android SDK35, my own experiences and success:
After the Unity-CVE incident I installed Unity2022.3.62f2 and also Android SDK36 and … IT JUST WORKED! I did remove all previous Android SDKs before doing that though.