Setereservasjonen for denne togvognen mangler.

It is a compound noun plus a definite ending:

• sete = seat
• reservasjon = reservation
• setereservasjon = seat reservation
• setereservasjonen = the seat reservation

The ending -en marks the noun as definite singular.
Also, in Norwegian compounds, the last part usually determines the gender, so because reservasjon is an en-word, the whole compound is too.

Because Norwegian usually writes compound nouns as one word.

So:

• tog = train
• vogn = carriage / car / wagon
• togvogn = train carriage / railcar

This is very normal in Norwegian. English often separates similar expressions, but Norwegian prefers compounds.

Because after denne, Norwegian normally uses the noun in the definite form.

So:

• en togvogn = a train carriage
• togvognen = the train carriage
• denne togvognen = this train carriage

This is a very common pattern:

• denne bilen = this car
• dette huset = this house
• disse bøkene = these books

Because togvogn is a common gender noun (an en-word).

• en togvogn → denne togvognen
• et hus → dette huset

So:

• denne = for singular common gender
• dette = for singular neuter
• disse = for plural

Mangler is the present tense of å mangle.

In this sentence, it means something like:

• is missing
• is absent
• is not there

So Setereservasjonen ... mangler means that the seat reservation is missing.

A useful thing to know is that å mangle can sometimes correspond to English to lack, but in sentences like this, English usually translates it more naturally as is missing rather than lacks.

It is not actually the very end because Norwegian likes verb-second word order in main clauses.

Here the structure is:

• Setereservasjonen = subject
• mangler = verb
• for denne togvognen = prepositional phrase

So the verb comes in the second position, which is normal in Norwegian main clauses.

Compare:

• Setereservasjonen mangler.
• I dag mangler setereservasjonen for denne togvognen.

In the second example, I dag takes first position, so the verb still stays second.

It means for this train carriage or for this railcar.

The preposition for connects the seat reservation to the specific carriage it belongs to. So the sentence is talking about the reservation information associated with this particular carriage.

It is singular definite: the seat reservation.

Here are the forms:

• en setereservasjon = a seat reservation
• setereservasjonen = the seat reservation
• setereservasjoner = seat reservations
• setereservasjonene = the seat reservations

So in your sentence, it refers to one seat reservation system/entry/reservation notice, not several.

No. Togvogn means a carriage, coach, or car of a train, not the entire train.

So:

• tog = train
• togvogn = one carriage/car of the train

That is why the sentence is about seat reservation for this carriage, not necessarily for the whole train.