I have the same error message. I made a huge map with tons of polygons, though, so I assumed it was somehow my fault. But the error message gives me no understanding of why my map has been rejected. I painstakingly spent hours remaking every polygon so that they don't overlap, still with no success.

Edit: I made a bunch of test maps with different polygons of various sizes and locations. I noticed that I can publish the map early and republish over and over again every time I add a new polygon because if I do too many in a row, one in inevitably causes it to fail and be unable to save. My impression is that polygons that are too big tend to fail for whatever reason, so I've just had to remake my map with progressively smaller and smaller polygons which is very inconvenient.

For example, selecting all of New Zealand was too many locations to fit into one polygon and caused it to fail to publish. You'd think that someone out there would've made a custom map with New Zealand at some point, no clue why that would fail. But splitting it into two polygons was enough to publish a map with New Zealand. Not sure if this is uncovering an existing bug, or just a design limitation that hasn't been stated by the website?

As a counterpoint, I was able to make a map with only one polygon, just covering all of North and South America in a single big rectangle. Certainly that's more locations than New Zealand, yet a North-South America map was possible with one polygon.