Learn Hardcore Norwegian: Meldingen har ikke kommet ennå.

ikke … ennå = “not yet.” This is the most standard written Bokmål choice.
enda can also mean “yet” in speech: Meldingen har ikke kommet enda. It’s more informal; ennå is preferred in writing.
fortsatt/fremdeles = “still” (affirmative): Han er fortsatt/fremdeles her.
For “no longer,” use ikke … lenger: Han er ikke her lenger.
Note: enda also means “even” as an intensifier: Det er enda bedre (“That’s even better”).

Why is ikke placed after har?

Norwegian main clauses follow the V2 rule: the finite verb is in second position. In Meldingen har ikke kommet ennå, the finite verb is har, so ikke comes after it.

Subordinate clause: … at meldingen ikke har kommet ennå (no V2; ikke precedes the verb there).

Why use the present perfect (har kommet) instead of the present or simple past?

har kommet expresses a result up to now and pairs naturally with ennå (“yet”).
Simple past states a finished past event: Meldingen kom ikke i går (“The message didn’t arrive yesterday”).
Present like Meldingen kommer ikke ennå sounds odd unless you mean a planned future arrival time (“It won’t be coming yet”).

Can I say er ikke kommet ennå instead of har ikke kommet ennå?

It’s possible but more formal/old-fashioned. With intransitive motion/change verbs, Bokmål allows both auxiliaries:

Neutral/most common today: har kommet.
Formal/literary or fixed expressions: er kommet (e.g., Våren er kommet = “Spring has arrived”). For a message, stick with har in everyday language.

What is kommet exactly?

It’s the past participle of komme (“to come”). Principal parts: komme – kom – har kommet.

No agreement: participles don’t change for gender/number after har.
In Nynorsk the participle is kome: har kome.

Why the definite form Meldingen and not En melding?

Norwegian usually uses the definite form for a specific, known/expected item: Meldingen = “the message (we’re both thinking of).”
Saying En melding har ikke kommet ennå is unusual unless you mean “Some/any message hasn’t arrived yet,” which is not the typical idea.

Is meldinga also correct?

Yes. Melding is a feminine noun. In Bokmål you can use either the masculine pattern (en melding – meldingen) or the feminine pattern (ei melding – meldinga). Both are correct Bokmål.

Can I move ennå and ikke around?

Yes, for emphasis or style:

Neutral: Meldingen har ikke kommet ennå.
Slightly more formal/emphatic: Meldingen har ennå ikke kommet.
Fronted adverb (keep V2): Ennå har meldingen ikke kommet.

How would I ask “Has the message arrived yet?”

Har meldingen kommet ennå?
Short answers: Ja, den har (allerede) kommet. / Nei, ikke ennå.
“Already” is allerede: Har meldingen allerede kommet?

How is this sentence pronounced?

Approximate East-Norwegian:

Meldingen: “MEL-ding-en” [ˈmɛl.dɪŋ.ən]
har: “har” [hɑːr]
ikke: “IK-ke” [ˈɪkːə]
kommet: “KOM-met” (long m) [ˈkɔmːət]
ennå: stress on the second syllable: “en-NÅ” [ɛnˈnoː]
Tip: Say the double consonants long (in ikke, kommet).

Can ennå mean “still” in affirmative sentences?

Yes, in formal/neutral style: Det er ennå tid (“There is still time”). In everyday speech, fortsatt or fremdeles is more common: Det er fortsatt/fremdeles tid.

Which pronoun do I use to refer back to meldingen?

Use den (common gender): Har du fått meldingen? Den har ikke kommet ennå.

If I mean a text message specifically, is melding the right word?

Yes, melding commonly refers to a text message. You can be explicit with tekstmelding or SMS.

Tekstmeldingen/SMS-en har ikke kommet ennå.

Should I add something like inn or fram/frem?

You can, to be more specific:

kommet inn = “come in” (e.g., into your inbox): Meldingen har ikke kommet inn ennå.
kommet fram/frem = “arrived (at the destination)”: Pakken har ikke kommet fram ennå.
Both fram and frem are accepted in Bokmål.

What would this look like in Nynorsk?

Meldinga har ikkje kome enno.
(Note: ikkje = “ikke”, kome = “kommet”, enno = “ennå”.)