AndroidStudio SDK directory does not exists
Right click your project and select 'Open Module Settings' under SDK Location put your location for your SDK.
paste in /Users/AhmadMusa/Library/Android/sdk
Clean and rebuild your project
Update
Try to delete your local.properties file and create a new one, but do not check it into version control.
Right click top level of project and Create new file 'local.properties'
then add: sdk.dir=/Users/AhmadMusa/Library/Android/sdk
Clean and build
I finally find this file on the disk.The 'local.properties' file in The Android studio is not which you modify.see the picture,so you can modify this line 'sdk.dir' to your dir of sdk.Remember not in the android studio.
This is a problem when you open the project incorrectly. open the project do not import the project