What propaganda technique is Napoleon using to get the animals to believe that Snowball is the cause of all of Animal Farm’s problems?

The specific propaganda technique that Napoleon uses to manipulate the animals into believing that Snowball is a vengeful enemy of the farm and the cause of all the farm's problems is scapegoating. Scapegoating is a popular propaganda technique where people deflect blame from themselves and attribute it to others. Napoleon utilizes scapegoating by blaming Snowball for the destruction of the windmill instead of taking responsibility for its collapse and admitting that there were faults in its construction. Anytime the farm experiences issues or Napoleon makes a catastrophic political mistake, he blames Snowball for it. By using Snowball as a scapegoat, Napoleon not only shifts the blame from himself but also continues to paint Snowball as the enemy. The animals are led to believe that Snowball is dedicated to the fall of Animal Farm, while Napoleon is an honorable, courageous leader. Napoleon also utilizes the propaganda technique of name-calling by referring to Snowball as a "traitor" and "criminal." Napoleon's name-calling propaganda allows him to tarnish and ruin Snowball's reputation, as the other animals begin to perceive him as the ultimate enemy of the farm.

This particular technique is called scapegoating, and it involves incorrectly placing the blame on Snowball for the farm's problems. Snowball, therefore, becomes the focus of unwarranted negative attention and treatment.
We see lots of examples of how Napoleon uses Snowball as a scapegoat. Probably the most effective instance comes from chapter 6 when the windmill is destroyed. Even though the destruction is clearly caused by bad weather, alongside the windmill's inadequate walls, Napoleon publicly blames Snowball. Specifically, Napoleon says that Snowball has destroyed the windmill in retaliation for his expulsion from the farm and because he does not want to see the farm succeed:

In sheer malignity, thinking to set back our plans and avenge himself for his ignominious expulsion, this traitor has crept here under cover of night and destroyed our work of nearly a year.

Through his use of scapegoating, Napoleon effectively blacken's Snowball's reputation. Over time, this leads the animals to denounce Snowball as an enemy of Animal Farm. Moreover, through scapegoating, Napoleon increases his own power and prestige because he becomes the focal point of the animals' positive attention and praise.