There is nothing major in the project, only the basics, IronSource, Payments, a couple of plugins. This started random at certain period of time but the code commits shows really basic changes during that time. I am investigating without much luck in what is the root cause.
If you reimport a script it may get to work again but will fail eventually. It happens at random after you change script code.
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:
I‘d go for deleting the Library folder to see if that fixes it.
If not, check if there is an updated patch level of your current Unity version and upgrade to that.
