I don't think you can use Arcade to visualize polylines as points. The error message seems to refer to the spatial reference. I did a simple test on a hosted feature that I published using the same spatial reference as the base map and it returned the point:
var geom = Geometry($feature);
Console('\ngeom:' + geom);
var line = Polyline(geom);
Console('\nline:' + line);
var cent = Centroid(line);
Console('\ncent:' + cent);
var pnt = Point(cent);
Console('\npnt:' + pnt);
return pnt;
Which yields:
geom:{"paths":[[[-8414259.482734734,697445.4680427602],[-8414258.288406167,697445.4680427602],[-8414258.288406167,697457.4113284288]]],"spatialReference":{"wkid":102100,"latestWkid":3857}}
line:{"paths":[[[-8414259.482734734,697445.4680427602],[-8414258.288406167,697445.4680427602],[-8414258.288406167,697457.4113284288]]],"spatialReference":{"wkid":102100,"latestWkid":3857}}
cent:{"x":-8414259.482734734,"y":697445.4680427602,"spatialReference":{"wkid":102100,"latestWkid":3857}}
pnt:{"x":-8414259.482734734,"y":697445.4680427602,"spatialReference":{"wkid":102100,"latestWkid":3857}}
So it returns the point, but the symbology will no be what you want (they will remain lines, and you will have an object for your symbology that you cannot actually use):