Run to the very same error a while ago. This worked for me.
From the Android Studio 3.6 new features blog post: New location to toggle Gradle's offline mode To enable or disable Gradle's offline mode, first select View > Tool Windows > Gradle from the menu bar. Then, near the top of the Gradle window, click Toggle Offline Mode Gradle offline button in the Gradle panel
Source: Cannot enable Gradle's offline mode on Android Studio 3.6
