This will not work on Windows CMD because the single quotes are not interpreted as combining all the characters together into one string. This is particularly a problem for JSON because commas are treated the same as spaces for parsing delimiters. You have to use the double-quote version, or use Powershell instead.
I suppose he's not using Windows CMD because his original command starts with "./" which wont work in it.
AFAIK the first example won't work in PowerShell either, just the other option but instead of
"{\"fee_rate\":20}", should be:
'{\"fee_rate\":20}' in PowerShell.
It's better if he mentioned his OS, right?