Learn Russian: Всё нормально, не беспокойся.

Всё means 'everything' and is used for concepts and abstract situations. Все means 'everyone' or 'all' (for people or plural objects). They look similar, but mixing them up is a common B1 trap. Here, you are saying 'everything is fine,' so you need the neuter singular pronoun всё.

Why do we use the adverb-looking word нормально instead of an adjective like нормальное?

In Russian, sentences that describe a general state, condition, or environment use 'category of state' words. These look exactly like short adverbs ending in -о. When using всё as the subject, it pairs with these state words rather than full adjectives. You'll see the exact same pattern in phrases like всё хорошо (everything is good) or всё понятно (everything is clear).

Why is the imperative не беспокойся in the imperfective aspect?

In Russian, negative commands (telling someone not to do something) almost always take the imperfective aspect. Even if you are telling them not to worry right now in this specific moment, the negation не strongly prefers the imperfective verb беспокоиться.

How would I use the так ... что (so ... that) structure from this lesson to say 'Everything is so fine that you shouldn't worry'?

While you could technically piece together Всё так нормально, что не беспокойся, it sounds quite clunky. A much more natural, native-sounding way to express this degree would be: Всё настолько хорошо, что тебе не о чем беспокоиться (Everything is so good that you have nothing to worry about).