Check your [sdk directory]/platform-tools directory and if it does not exist, then open the SDK manager in the Android Studio (a button somewhere in the top menu, android logo with a down arrow), switch to SDK tools tab and and select/install the Android SDK Platform-tools.
Alternatively, you can try the standalone SDK Manager: Open the SDK manager and you should see a "Launch Standalone SDK manager" link somewhere at the bottom of the settings window. Click and open the standalone SDK manager, then install/update the
"Tools > Android SDK platform tools".
If the above does not solve the problem, try reinstalling the tools: open the "Standalone SDK manager" and uninstall the Android SDK platform-tools, delete the [your sdk directory]/platform-tools directory completely and install it again using the SDK manager.
Hope this helps!