Could try a reimport all, or clear the IL2CPP cache in your project, or even remove and re-add the ECS and other packages to force a full recompile.
Otherwise… isolate!!
How to troubleshoot build failures:
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 third-party libraries such as Firebase.
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:
Recently (circa July 2022) there 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…):
Android Gradle errors and other related stuff: