I'd look at the unique values in that column and see if there are text values there. For example, 'N/A' can't be converted to a floating decimal because it is text. The "Failed to parse text" in the error leads me to believe there are text values for that field.
@pauljames
You may need to put a filter before this alter step to filter out rows that are text before you do the alter. So you probably need to first get a list of all unique values and see if there are a lot of text values
