Prefixes will not work

[RileyRides]

Prefixes that I set in the config will not work. (Message Content Intent is turned on and have tried turning it off saving and turning it back on and saving but still no luck.) Bot plays fine with @mention I just would like to be able to set a prefix to make things more simple. Also when I run the bot I get this error:
"Failed to write new config options to config.txt:java.nio.file.AcessDeniedException:config.txt
and it says to make sure my files are not on my desktop but they are all in a folder in my Documents. Any ideas on how to fix this?

Thank you in advance!