U fotokopirnici opet fotokopiram papire, jer mi se printer kod kuće stalno gasi.

Because u meaning in/inside (location, not motion) requires the locative case.

• Nominative: fotokopirnica (a copy shop)
• Locative: u fotokopirnici (in the copy shop)
  If you were talking about motion into the place, you’d usually use u + accusative: Idem u fotokopirnicu. (I’m going to the copy shop.)

Opet means again. Here it implies: I’m photocopying again (as usual / once more).
Word order is flexible; these are all possible with slightly different emphasis:

• U fotokopirnici opet fotokopiram papire. (Again—at the copy shop—I'm copying.)
• U fotokopirnici fotokopiram papire opet. (Again, at the end; more “afterthought”.)
• Opet fotokopiram papire u fotokopirnici. (Again is the main point.)

Because papire is the accusative plural, used for the direct object (what you are photocopying).

• Nominative plural (subject): papiri
• Accusative plural (object): papire
  Example contrast:
• Papiri su na stolu. (The papers are on the table.)
• Fotokopiram papire. (I’m photocopying the papers.)

Because jer introduces a dependent clause giving a reason (because). In Croatian, it’s standard to put a comma before jer in this use:

• Main clause, jer
  • reason clause.

This is a common Croatian structure combining:

• mi = to me (dative pronoun; indicates it affects me / happens “to me”)
• se = reflexive marker used with certain verbs to form an intransitive or “happening” meaning

So gasi se = turns off (by itself), and gasi mi se = keeps turning off on me / keeps shutting down (and it’s a problem for me).

Croatian also has pisač (printer), but printer is extremely common in everyday speech and is fully normal. You could say:

• ... jer mi se pisač kod kuće stalno gasi.
  It sounds slightly more “Croatian,” but both are natural.

Kod kuće literally means by/at home, i.e., at home (location).
Kući usually means (to) home (direction), like “going home,” and can also appear in some “at home” contexts in speech, but the clean contrast is:

• Kod kuće = at home
• Idem kući = I’m going home

Gasiti (without se) is usually to turn off (something) (transitive):

• Gasim printer. = I’m turning off the printer.

Gasiti se means to turn off / shut down (intransitive, “it happens”):

• Printer se gasi. = The printer is shutting off.
  With stalno, it strongly implies it keeps shutting off on its own (or unintentionally).

It’s a very natural order, but Croatian allows alternatives for emphasis:

• ... jer mi se printer stalno gasi kod kuće. (puts emphasis on “at home” later)
• ... jer mi se kod kuće stalno gasi printer. (brings “at home” earlier)
  The core grammar stays: mi se
  • subject (printer) + adverbs + verb (gasi).