Failed to find Build Tools revision 29.0.0 - Android Builds

You should download Android SDK Build-Tools.

in Android Studio, go to Tools -> SDK Manager

in the left plane, select Appearance & Behavior -> System Settings -> Android SDK

now in the right plane Select SDK Tools after that in the bottom right corner there is checkbox called Show Pakage Details.

Expand the Android SDK Build-Tools and you can see the version of it.

Unselect the current version and Apply the Settings, You will ask to uninstall current version of build tools

And Select the version 29.0.0 and install it

For building a project, Unity3d uses a stable version of the Build Tools and not an rc (release candidate).

The Android SDK path is usually C:\Users\<username>\AppData\Local\Android\sdk\build-tools\ delete the 29.0.0-rc1 folder or move it out to anywhere. You could also remove the rc version of the SDK Build Tools via Android Studio's SDK manager.

Doing that the project should build successfully.