Learn Hardcore Finnish: Kannattaa ottaa sateenvarjo mukaan. - It is worth taking an umbrella with you.

ottaa mukaan: take something <a class="border-b border-dotted border-blue-400 dark:border-blue-500 cursor-pointer text-inherit no-underline" href="/grammar/finnish/postpositions/kanssa">with you (the act of including it as you go).
tuoda: bring (towards the speaker’s or target location).
viedä: take (away to another location). Examples: Tuo sateenvarjo mukaan (“bring the umbrella along [to here]”), Vie sateenvarjo mukaan (“take the umbrella along [to there]”), Ota sateenvarjo mukaan (neutral “take along”).

Here kannattaa means “it’s worth it / it’s advisable,” like English “you/one should.” So Kannattaa ottaa... ≈ “It’s a good idea to take...”. Note: kannattaa also means “to support (a team/idea)” and “to physically support,” but not in this sentence.

Where is the subject (“you/one”)? Why is there no pronoun?

This is a common subjectless, generic recommendation. It addresses anyone in the context (like English “you should” or “one should”). If you want to name the person, use a genitive “possessor”:

Minun kannattaa ottaa... = “I should take...”
Sinun kannattaa ottaa... = “You should take...”

Why is ottaa in the basic form (infinitive), not a conjugated form like otan?

Verbs like kannattaa are followed by the A-infinitive (dictionary form): kannattaa + ottaa. Think of it as “it’s worth (to) take.” You do not use ottamaan here.

Why is it sateenvarjo and not sateenvarjon?

In impersonal/necessive patterns like kannattaa + infinitive, the object is normally in the nominative when it’s a “total” object: kannattaa lukea tämä kirja, täytyy avata ovi.
But with a personal, finite verb you’d use the genitive/accusative: Otan sateenvarjon mukaan (“I’ll take the umbrella along”).
You may hear sateenvarjon after kannattaa in speech, but standard usage prefers nominative here.