This is happening for me in 1.4, but with proportional representation. I'm trying to represent plants proportionally based on feet/radius on a long field (which contains some nulls), and I get the same error as others mention, banner across the top of the tool with no other info apparent. The real problem is that we are not given enough information about why the tool is failing!...A wild guess in my case would be that the proportional symbology tool is not checking all the fields (there are like 20 in my case), doesn't find a numeric field type in the first few, and then assumes the tool won't work. Just a thought.