Learn Polish: Ochrona wpuszcza kibiców na te sektory. - Security is letting fans into these sectors.

The noun sektor is an inanimate object. In Polish plural forms, nouns are divided into masculine-personal (groups of people including at least one man) and everything else (non-masculine-personal). Since sectors are objects, they fall into the "everything else" category, so we use the pronoun te. We only use ci for masculine-personal nouns.

Is kibiców in the Genitive case here?

It looks like it, but it is actually the Accusative case! The verb wpuszcza (lets in) needs a direct object in the Accusative. For masculine-personal nouns like kibic (fan), the Accusative plural shares the exact same form as the Genitive plural, ending in -ów.

Why does the preposition na use the Accusative case (te sektory) instead of Locative?

The preposition na triggers the Accusative case when it describes motion or direction towards a destination (letting fans "into" or "onto" the sectors). If the fans were already sitting in the sectors, you would use the Locative case to describe their static location.

The word ochrona is singular. Does it refer to just one security guard?

No, ochrona is a collective noun. It is grammatically singular and takes the singular verb wpuszcza, but it translates to "security" as a whole team or staff, rather than an individual guard (which would be ochroniarz).