Which logical fallacy is committed in the following argument: Atheists have not proven that God does not exist?

The claim leans on the absence of disproof rather than on any positive evidence.

It notes only that atheists have not proven God does not exist.

From this lack of a disproof it treats God's existence as thereby supported.

But failure to prove a negative is not evidence for the positive.

Taking absence of refutation as proof is the appeal to ignorance (argumentum ad ignorantiam).