Ud839_miwok: Error:Could not download gradle-core.jar (com.android.tools.build:gradle-core:2.1.0): No cached version available for offline mode

as the error description says, " No cached version available for offline mode", so you need to turn on gradle and rebuild application, in order to do this: File-->Settings -->Build, Executions, Deployment --> Build Tools --> Gradle --> uncheck the "offline work" --> click on the apply button --> rebuild your application

have fun

@elyashadizadeh Oh thank you very much! (After I Unchecked the "Offline work", Rebuilding application didn't help, but i imported the project again and this time instead of giving error some update of SDK operation ran and opened the project). Thank you once again.

If working on react-native, and you are trying to run the android build, try changing the class path in your build.gradle path from classpath 'com.android.tools.build:gradle:3.1.4'
to classpath 'com.android.tools.build:gradle:3.0.1'

@elyashadizadeh Thanks a lot :)

i change the classpath from 'com.android.tools.build:gradle:3.0.1 to 'com.android.tools.build:gradle:3.5.3' in the build.gradle

As in my case, it is not the problem with the Miwok app, it simply the project is not in sync with the Gradle.
To make it syn you have to do the following-->
1.Go to Gradle(View-->Tool Window-->Gradle)
2.Toggle the Offline mode.

3.Go to File
4.Click Sync Project with Gradle File.
Make sure you are connected to the internet to download the required file, be patience if you have a slow internet connection, bz it would take some time to complete.
You are done, enjoy.