Why did the Gradle sync fail, with the reason being that compileSdkVersion was not mentioned?

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The Gradle sync failed because the compileSdkVersion was not mentioned in the project's build.gradle file. This is necessary because the compileSdkVersion specifies the maximum version of the Android SDK that the app can use, and it needs to be specified in order for Gradle to know which SDK version to use when building the app. Without this information, Gradle is unable to resolve dependencies and compile the code.