compileSdkVersion is not specified. Please add it to build.gradle

[3xscola]

* What went wrong:
A problem occurred configuring project ':christian_picker_image'.
> Failed to notify project evaluation listener.
   > Cannot invoke method substring() on null object
   > compileSdkVersion is not specified. Please add it to build.gradle

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
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* Get more help at https://help.gradle.org

BUILD FAILED in 19s
Exception: Gradle task assembleDebug failed with exit code 1