Das ist genau das was ich sehen wollte!

English Translation

This is exactly what I wanted to see!

Shouldn’t there be a comma before “was”?

Yes, there is a comma missing.

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The orthography reformation relaxed the comma rules and introduced circumstances where the comma has become optional.
I was close to thinking that it’s optional here, but after reading the rules, it’s clear that this is a relative clause where the comma is still mandatory.

You can read the rules here: Kommaregeln der deutschen Sprache (Stand von 2006) – Wikipedia
The section about Nebensätze does not list this specific example (“das, was”), but it also does not explicitly mention exceptions for relative clauses.

This other site has a “das, was” example: Kommaregeln einfach erklärt - in 4 Schritten - phase6 Magazin

That said, many people are sloppy with the comma rules and put commas where there is a pause in speech, and in this sentence I would argue that there is no pause unless you want to emphasize. So that may explain why the author did not put a comma there.
But I agree, in a language learning app the sentences should be correct and not teach bad habits.