Some communist governments are authoritarian, all fascist governments are authoritarian, and not all authoritarian governments are fascists. The Soviet Union under its most authoritarian and repressive ruler fought and defeated fascism. So the Soviet Union was in the first category above: communist + authoritarian.

Fascism is more then authoritarianism. It is a set of explicitly right wing and explicitly anti-communist and anti-socialist policies, fascism is extremely pro-business. And it is explicitly racist and nationalistic.

The Soviet Union was explicitly pro-worker, it was explicitly communist, it was explicitly left-wing. Under its first ruler it was even explicitly internationalist (to the extend that the Internationale was their national anthem; But their second ruler was for sure a nationalist). There is no definition of fascism which includes the Soviet Union. If the Soviet Union were fascist, then fascism becomes a useless term completely synonymous with authoritarianism.