What doubts did the chiefs had about Ignosi? What did they demand?

The chiefs believed that Ignosi might very well have been tattoed later in life.... that he was a deceiver, thus, they required proof. The chiefs asked for a sign.

How know we that if we lift our spears it may
not be for a thief and a liar? It is a great matter, I say, of which
none can see the end. For of this be sure, blood will flow in rivers
before the deed is done; many will still cleave to the king, for men
worship the sun that still shines bright in the heavens, rather than
that which has not risen. These white men from the Stars, their magic
is great, and Ignosi is under the cover of their wing. If he be indeed
the rightful king, let them give us a sign, and let the people have a
sign, that all may see. So shall men cleave to us, knowing of a truth
that the white man's magic is with them."

"Ye have the sign of the snake," I answered.

"My lord, it is not enough. The snake may have been placed there since
the man's childhood. Show us a sign, and it will suffice. But we will
not move without a sign."