A problem occurred configuring project ':app'

[praba113]

What went wrong:
A problem occurred configuring project ':app'.

A problem occurred configuring project ':react-native-linear-gradient'.
failed to find target with hash string 'android-26' in: C:\Users\user\AppData\Local\Android\Sdk

Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

Total time: 22.96 secs
Could not install the app on the device, read the error above for details. Make sure you have an Android emulator running or a device connected and have set up your Android development environment:
https://facebook.github.io/react-native/docs/getting-started.html