If Nasus Guardian of Knowledge was at a battlefield and died but killed an enemy unit too, does his ability to channel 1 rune trigger?

No — Nasus's ability does not trigger.

When Nasus and an enemy unit die simultaneously (e.g., both take lethal damage in combat), Nasus leaves the board at the same instant the trigger condition ("when an enemy unit here dies") is met. Per Rule 383.2.c.2, a game object cannot evaluate its triggered condition if it leaves the zone its trigger is active from at the same time the condition is satisfied. Multiple FAQs confirm this: units that die simultaneously do not see each other die (FAQ #7692, #4493, #3223).

Nasus must be alive on the battlefield when the enemy unit dies for his ability to trigger. If he dies at the same time as the enemy, the trigger never hits the chain.