Learn Russian: Я ни с кем не поболтал на скамейке.

Adding по- to the imperfective verb болтать (to chat) creates the perfective verb поболтать. The по- prefix here indicates an action performed for a short, limited amount of time. So, it specifically means "didn't chat for a bit" or "didn't have a little chat."

Why is the phrase written as ни с кем instead of с никем?

In Russian, when a negative pronoun like никто (nobody) is used with a preposition like с (with), the preposition splits the pronoun in two and sits right in the middle. The word кем is the instrumental case of кто, which is required by the preposition с.

Why do we need не before the verb when we already have ни с кем?

Russian strictly requires a double negative. Whenever you use a negative phrase like ни с кем (with nobody / not with anybody), you must also make the verb itself negative by placing не right in front of it. Literally, the sentence translates to "I with nobody not chatted."

Why is на скамейке in the prepositional case rather than the accusative?

The prepositional case is used here because the sentence describes a static location: it tells us where the action failed to happen. If the sentence described motion directed onto the bench (for example, "I sat down on the bench"), you would use the accusative case instead.