why is the soviet union considered communist ?

hi so idk much about economic systems and politics bc i’m 14 but im actively trying to know more about these topics
last year, at school (im french btw) we worked on communism a bit and we were taught that communism according to karl marx is a society where the workers are supposed to rule and own the means of production, so how can a dictatorship match that definition ?
(and pls don’t tell me it’s in the name bc the nazis weren’t leftists even if they called themselves national socialists)