What you need to do is told by the script:
"Update your browser user agent variable, then sign back in to OF and update your session variables."
Go to OF web page, press F12 on your keyboard again, get the "hints" entry again, copy and paste the user-agent into the onlyfans-dl.py file, log out and back in from onlyfans, now press F12 again, copy the "sess" value from cookie string, then paste this value into the onlyfans-dl.py file and try using the script again.