A genocide is not defined by literal death of individuals but for killings to be part of a genocide they have to intentionally target a specific group and its culture.
The Nazi's Eastern front was explicitly a war to create "Lebensraum" (living space) which went with depopulating large areas. They also deliberately initially displaced and then systematically exterminated Jewish populations. They also targeted Sinti and Roma people. For Jews they also went to the lengths of deliberately destroying their cultural artefacts as well as works they claimed to be influenced by Jewish culture.
If you desperately want to pin a mass murder on historical communists, you probably want the Holodomor. But even the Holodomor seems to have at least in part been a case of administrative failure and apparatchiks not taking complaints seriously. But at least there are indicators of some level of malintent even if it may not have been intentionally genocidal.
Unlike the above, the treatment of the Uyghur Muslims does qualify as genocide under most definitions as it actively seeks out to erase culture and traditions even if the settling of Han Chinese people in Uyghur territory may not. But while it involves imprisonment, so-called reeducation and arbitrary arrests, it doesn't involve mass murder.
Note that what the Nazis did to Jews, Roma, Sinti and (to a lesser extent) Slavs wasn't unique in European history either (except for the technology available to them), but Europeans previously only had done this to populations in Africa or the Americas. From the victim's perspective the distinction between fascism and colonialism is at most times completely arbitrary.
