Learn Hardcore Spanish (Spain): El enchufe está detrás del sofá.

Spanish uses estar for the location of physical objects. Since the sentence states where something is, está is correct. Use ser only for events (e.g., La reunión es en la sala) or inherent traits.

Why do we say detrás del and not detrás de el?

Because Spanish contracts de + el → del. Other articles don’t contract: de la, de los, de las. Also, the contraction never happens with the pronoun él (see below).

Does detrás always need de?

When followed by a complement, yes: detrás de + noun/pronoun (e.g., detrás del sofá, detrás de mí). On its own, you can say simply Está detrás if the reference is understood.

What’s the difference between detrás and atrás?

detrás de introduces what something is behind: detrás del sofá.
atrás is an adverb meaning “at the back/behind/backwards” and typically doesn’t take a complement in Spain: Mira atrás.
In much of Latin America people do say atrás de, but in Spain detrás de is the standard.

Is acceptable in Spain?