I originally used a circle generator kml produced online (https://kml4earth.appspot.com/circlegen.html)–this was referenced in a GE forum post. Another said to use the circle tool in the ruler. The OP asked specifically about a polygon. I got the same error with both. I also just tried drawing a square polygon inside the circle, to compare the files. That file does have a Polygon tag, and LinearRing. The ruler circle file doesn’t have a Polygon tag, and uses LineString. I tried replacing the coordinates from the circle file into the square file just to see, but that didn’t work either.
This is the version run from the generator, revised per the instructions to make the radius 7.5 in GE.
<?xml version="1.0" encoding="UTF-8"?>
Circle
radius 7.5 mi
ff0000ff
2
1
-121.9300191099898,37.60671471165103,0 -121.9315165352067,37.62292477690493,0 -121.936035974074,37.63877609639133,0 -121.9434782540387,37.65391424435877,0 -121.9536785828138,37.66800053408341,0 -121.9664100991427,37.68071963009751,0 -121.9813888507189,37.69178665188468,0 -121.9982800962749,37.70095360505985,0 -122.016705794911,37.70801499034992,0 -122.036253114468,37.71281245887155,0 -122.056483763287,37.71523840388709,0 -122.0769439270961,37.71523840388709,0 -122.0971745759151,37.71281245887155,0 -122.116721895472,37.70801499034992,0 -122.1351475941081,37.70095360505985,0 -122.1520388396641,37.69178665188468,0 -122.1670175912403,37.68071963009752,0 -122.1797491075692,37.66800053408341,0 -122.1899494363443,37.65391424435876,0 -122.197391716309,37.63877609639133,0 -122.2019111551763,37.62292477690493,0 -122.2034085803932,37.60671471165103,0 -122.201852495229,37.59050811879209,0 -122.1972796085517,37.57466690795776,0 -122.1897938419495,37.55954460671386,0 -122.1795638516837,37.5454784937492,0 -122.1668191345942,37.53278211175164,0 -122.1518448159863,37.52173832300449,0 -122.1349752432974,37.5125930575328,0 -122.1165865317413,37.50554988755127,0 -122.0970882270262,37.50076554341755,0 -122.0769142656054,37.49834646569506,0 -122.0565134247777,37.49834646569506,0 -122.0363394633568,37.50076554341755,0 -122.0168411586417,37.50554988755127,0 -121.9984524470857,37.5125930575328,0 -121.9815828743968,37.52173832300449,0 -121.9666085557888,37.53278211175164,0 -121.9538638386993,37.5454784937492,0 -121.9436338484335,37.55954460671386,0 -121.9361480818314,37.57466690795776,0 -121.9315751951539,37.59050811879209,0 -121.9300191099898,37.60671471165103,0