Learn Spanish (Latin America): ¡Qué alivio que cancelen el proyecto! - What a relief that they are canceling the project!

Because the first part of the sentence expresses an emotion or judgment. The structure ¡Qué + noun + que... sets up an emotional reaction (in this case, relief or alivio). When you react to a situation rather than just objectively declaring it as a fact, the verb in the following clause must be in the subjunctive.

Why is it just Qué alivio and not Qué un alivio to match "What a relief"?

In Spanish exclamations starting with Qué, you never use the indefinite articles un or una. You simply attach the noun or adjective directly to Qué. Adding un here is a very common pitfall for English speakers!

Who is the subject of cancelen in this sentence?

It is conjugated in the third-person plural ("they" or ellos). In this context, it is an impersonal "they," referring to whoever is in charge of the project (like a committee or the bosses). Spanish frequently uses this "they" form when the specific people doing the action aren't identified.

The English translation says "they are canceling". Could I say estén cancelando instead?