Although there were a number of tactical errors, Hitler’s three biggest mistakes in Operation Barbarossa were these IMHO:
1) Failure to take Moscow and Leningrad early in the campaign when they had the chance.
2) Failure to provide winter clothing for the troops.
3) Failure to treat locals better than the Communists did who initially welcomed the Wehrmacht as liberators.
Especially the Ukrainians, they would have joined the Nazis in fighting the Soviets. All Hitler had to do was to set up a puppet Ukrainian regime, along the lines of say, Slovakia.
And by taking Moscow, it probably would have lead to Stalin's removal, and perhaps a complete overthrow of the Bolshevik regime, by the Russians themselves.