Installed Build Tools revision 31.0.0 is corrupted. Remove and install again using the SDK Manager解决方法
//通过Android studio的SDK Manager卸载然后重新下载build-tools\30.0.0和build-tools\31.0.0，确保最新的
//下载下来的build-tools\31.0.0版本是缺少dx.bat文件和lib\dx.jar文件的
//所以将build-tools\30.0.0版本的dx.bat和dx.jar分别拷贝一份到build-tools\31.0.0对应的目录下即可解决问题
将androidSdk\build-tools\30.0.0\dx.bat 拷贝一份到 androidSdk\build-tools\31.0.0\dx.bat
将androidSdk\build-tools\30.0.0\lib\dx.jar 拷贝一份到 androidSdk\build-tools\31.0.0\lib\dx.jar