The former Rangers and Motherwell forward is the first confirmed withdrawal from a squad of 28 players selected for the friendly against Estonia at Pittodrie on Wednesday night.
Leeds revealed McCormack had joined up with the squad but had then been sent home with "flu symptoms".
McCormack wrote on his Twitter account: "Gutted to be pulling out the Scotland squad ill... Now just rest and get fluids in me and be ready for wolves on Saturday!!"
The 26-year-old, who scored in a 3-1 friendly win over Australia in August, was recalled to the squad by new Scotland manager Gordon Strachan days after netting in an FA Cup victory over Tottenham.
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- Scotland squad