Learn Italian: Ti raccomando di prendere lo sciroppo se la pressione è alta. - I advise you to take the syrup if your blood pressure is high.

If you were speaking directly to the person, you would use the imperative: "Prendi lo sciroppo!" ("Take the syrup!"). To turn this into an indirect command, Italian uses a reporting verb like raccomandare (to recommend) or dire (to tell), an indirect object (ti), and connects them to the action using the preposition di followed by the infinitive verb (prendere).

Is ti a direct or indirect object pronoun here?

It is an indirect object pronoun. The verb raccomandare takes an indirect object when you are telling someone to do something. The full structure is raccomandare a qualcuno di fare qualcosa (to recommend to someone to do something). Here, ti means to you.

Why does la pressione translate to "your blood pressure" instead of just "the pressure"?

In health contexts, la pressione is the standard everyday shorthand for la pressione sanguigna (blood pressure). Also, as a quick refresher: Italian typically uses the definite article (la) instead of a possessive adjective (like tua) when referring to body parts or physical conditions, because the context already makes it obvious whose body is being discussed.

Could I use che instead of di to say "I recommend that you take..."?