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I mean, if damage is countable, it should be

Deal 6 damages.

If it’s not countable, then this sentence should be wrong.

Such as saying something like

I drank 5 water.

So... am I missing something here?

It's domain specific, and not something that would be said outside the context of a game like this.

It's almost certainly an elided form of the following:

Deal 6 points of damage.

(And damage here is a mass noun.)

In the same way that headlines take liberties with the omission of articles and other grammatical structures, so too is this game using a shortened form of English that's understood in its own context. (With that font size, it looks like the full sentence might not fit within the space allowed by the card.)

Generically, because it's established gaming jargon. While the answer by Jason Bassford is almost certainly correct about the origins of this particular bit of jargon, it's gotten to the point now that it's just accepted jargon, so it's what almost everybody uses.

In a number of cases, the jargon for a particular domain is essentially a distinct grammatical and lexical dialect from the base language it's used in, and should be analyzed as such since it quite often just doesn't make sense otherwise.

In this particular case, the construct [verb] [number] [attribute or property] is in widespread use in many types of games as a way of concisely expressing a numerical change in state of some specific value within the context of the game. The verb indicates the particular direction of the change (positive or negative), the number is largely universally a positive, and the attribute or propperty indicates what is being changed.

So, in your example, 'deal 6 damage' means that whatever entity is being targeted takes six points of damage, but expresses that without needing nearly as many words.

That kind of concise communication gets really important in a lot of cases because space is often limited when relaying information like this, so fewer words means you can use a bigger font, and therefore make it more easily readable (this is less of an issue in a digital context though than it is with physical games).

As mentioned above, the origins of this phrase are almost certainly exactly what Jason Bassford outlined in his answer. Exactly pinpointing it's origin is somewhat difficult, but I'd be willing to bet that it developed first as verbal shorthand among players of tabletop RPGs (like Dungeons & Dragons) and then got slowly inherited by other gaming contexts (many gamers tend to play more than one type of game). It's long-since become standard phrasing in TCG's and CCG's, likely because of Magic the Gathering (which goes a step further and uses similar phrasing to indicate changes in certain non-numeric properties as well), and that's probably where the usage in your particular case came from (pretty sure the picture is a card from Hearthstone, which took heavy inspiration in a lot of ways from MtG, just like most other TCG type games).

Many games consider damage a unit. Just as we would say:

6 kilogram (instead of kilograms)

These games say:

6 damage

It depends on how the word is defined.