Learn Hardcore Norwegian: Hennes utdanning er innen helse, men hun jobber i en butikk som mangler folk. - Her education is in health, but she works in a store that lacks staff.

hennes is the non‑reflexive 3rd‑person possessive (“her”/“hers”), used when the possessor is not the subject of the same clause, or when you front the possessive.
sin/si/sitt/sine is reflexive and is used when the possessor is the subject of the same clause, and it normally follows the noun: utdanningen sin.
In this sentence, the clause subject is the whole noun phrase Hennes utdanning, not hun, so reflexive sin doesn’t fit here. The neutral alternative would be: Utdanningen hennes er innen helse.

Examples:

Hun har utdanningen sin innen helse. (reflexive, possessor = subject)
Hennes utdanning er innen helse. (fronted, non‑reflexive)
Utdanningen hennes er innen helse. (postposed, non‑reflexive, very common)

Is Hennes utdanning er innen helse natural, or is there a better way to say it?

It’s grammatical and a bit formal/emphatic. Two very natural alternatives:

Utdanningen hennes er innen helse. (neutral)
Hun er utdannet innen helse. (very idiomatic) You can also say: Hun har utdanning innen helse or more specifically Hun har utdanning i helsefag.

Why innen helse and not i helse?

innen means “within (the field of)” and is the default preposition for study/work domains: innen økonomi, innen IT, innen helse.
i can work if you name a concrete sector/institution: i helsesektoren, i helsevesenet.
innenfor also exists (“within”), but innen is more idiomatic in domain phrases. Avoid på helse in standard Bokmål for this meaning.

What exactly does helse refer to here? How is it different from helsevesen or helsefag?

helse = health/healthcare as a broad field.
helsevesenet = the healthcare system (institutions/services).
helsefag = healthcare subjects/health sciences (as study programs). So, innen helse is general; you could specify with i helsevesenet (in the system) or i helsefag (in the academic track).

Why is there a comma before men?

Norwegian normally uses a comma before coordinating conjunctions like men, og, for, eller when they link two main clauses. Here we have: main clause 1 (Hennes utdanning er …), comma, main clause 2 (men hun jobber …). Word order after men stays normal main‑clause order: hun jobber …

What’s the difference between hun jobber i en butikk and hun jobber i butikk? And what about på?

i en butikk = in a specific shop (an instance).
i butikk (no article) = in retail (the line of work) in general.
på is common with named workplaces/institutions: hun jobber på IKEA / på sykehuset. But på en butikk is not standard in Bokmål (though some dialects say it). For an unnamed store, use i (en) butikk.

How does the relative pronoun som work in en butikk som mangler folk?

som is the general relative pronoun for “that/which/who”.
Here, som is the subject of the relative clause, referring to butikk: subject (som) + verb (mangler).
When som is the subject, you cannot drop it. So you need som in this sentence.

Does mangler folk mean “misses people” emotionally? How is mangle different from savne?

mangle = “to lack / be missing (have a shortage of)”: Butikken mangler folk = the shop is short‑staffed.
savne = “to miss (emotionally)”: Hun savner familien. So here it clearly means a shortage, not an emotional sense.

Why use folk and not personer or ansatte?

folk is common, informal, and generic “people”; with workplaces it often implies staff: “short‑staffed.”
personer is more formal and countable (“persons/individuals”), less idiomatic here.
ansatte (“employees/staff”) is the clearest if you want to make it explicit: en butikk som mangler ansatte. Default everyday phrasing is mangler folk.

Do I need an article with folk?

No. folk is used without an article in the indefinite: mangler folk, det er mye folk her. The definite forms exist but have special uses (folket = “the people (as a body/nation)”, folkene = “the people (individuals)”), and folka is colloquial. For “some people” you can add noen: mangler noen folk (though mangler folk is more idiomatic for “short‑staffed”).

What gender is butikk, and what are its forms?

butikk is masculine in Bokmål:

Indefinite singular: en butikk
Definite singular: butikken
Indefinite plural: butikker
Definite plural: butikkene

Could the som‑clause be read as “lacking customers” instead of staff?

By default, mangler folk is taken as “short‑staffed.” If you want “lacking customers,” say mangler kunder. If you want to be crystal‑clear about staff, say mangler ansatte or er underbemannet.

Can I use der or hvor instead of som?

Yes, but the structure changes:

i en butikk der de mangler folk (where they lack people)
i en butikk hvor de mangler folk Here, der/hvor introduces a place‑relative clause, and you provide a new subject (de). With som, the shop itself is the subject of the clause: en butikk som mangler folk.

Is it utdanning or utdannelse?

Both exist in Bokmål. utdanning is more common and is the recommended everyday form. utdannelse is also correct but feels a bit more formal/old‑fashioned to some speakers. In this sentence, utdanning is the natural choice.

What’s the word order inside the som‑clause?

In relative/subordinate clauses, you don’t use main‑clause V2 inversion. The order is Subject–Verb–Object/Other:

som (subject) + mangler (verb) + folk (object). That’s why we get som mangler folk, not something like som folk mangler in this case.