Dania Wajak please try to add Language Cloud from the Project Setting after the project is created and you are in Editor mode. This way Trados Studio knows what language pair you have on the project and will provide you the Arabic - English engine. I have tried this and it works.
Let me know if this works for you.
This happens because the default language in File- Options - Language Pairs is different than the one on your project.
If you will be using all the time Arabic - English, as a temporary workaround, please go to the main menu File -> Options -> Language Pairs and add Arabic - English as your first default language pair.
Radu Boloveschi# Thank you, It worked out perfectly.