hi! there's lots of room for more input on this to represent any populations who did believe the Earth was flat, but meanwhile, check these out:
FAQ background on The flat earth myth
similar question When people thought the Earth was flat, did they think the moon was a 2D circle in the sky? - most of the thread is debating the "flat Earth" thing, but keep looking, there are a few comments on the moon
Why did people think the earth was flat when they could see the moon?