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The problem is that the gradlew file in the android folder isn't executable anymore because you cloned it from a git repo.
Basically, when you do react-native run-android, It does a lot of thing which includs running commands for you such as cd android && ./gradlew installDebug. right there, it tries to execute the gradlew, but can't, cause the file isn't executable. which is why you get that error.
just cd into the android folder, and do chmod +x gradlew.