I had the same problem on Windows, and I found the solution in Volodymyr's comment in Error:Unable to locate adb within SDK in Android Studio
1. Make sure that 'Android SDK Platform-Tools' is installed:
Tools > SDK Manager > SDK Tools (Tab)
2. Copy path of 'adb.exe' file. Usually here:
C:\Users\Username\AppData\Local\Android\Sdk\platform-tools
3. Open Enviroment Variables in Windows:
'Window + R' > and type 'SystemPropertiesAdvanced'> Enviroment
Variables...
4. Create new system variable with name 'PATH' and value of your adb.exe path like that 'C:\Users\Username\AppData\Local\Android\Sdk\platform-tools'