There are thousands of maps with no "mountains", though they may have some craters and crevasses. You just have to look through a few dozen of the "relatively flat" colony sites to line up a few.
Here. Try this. 62Sx53E.
That took me about 10 minutes of running through random possibilities to find, and that was because I was rejecting really low resources and the worst disaster climates.
Do quick searches using the default set up just to get a look at the map, then save the ones that meet your approval, to capture the coordinates.
Also, be careful of that word "mountains". "Mountainous" has a specific meaning in Surviving Mars. In practical terms it means a map that is cut up into many compartments (4 to 6+ zones) by extensive impassible terrain, which each require tunneling to access. Mountainous maps also tend to have at least one very high elevation area, although it may not be very large. What you are talking about in game terms is too much uneven or unbuildable terrain on an otherwise uncompartmented map.
