The only time that I can remember "many" people wearing tribal attire pointing AK47's at US servicemen was the battle of Mogadishu.
The movie Black Hawk Down was about that event. Several American soldiers were lynched by a tribal mob and their bodies were dragged through the streets.
Mogadishu is in Somalia, but it's right on the border with Kenya.
It occurred in 1993.
Obama visited Kenya in 1992 according to this article:
https://abcnews.go.com/Politics/barack-obamas-trips-kenya-now/story?id=32650996
I can't find any report about how long his visit lasted.