Critical: Changed my production app to use JWT and now getting OAU-251 error

I just changed my existing production app to use JWT and it's not working and I can't change back my auth type to user name. My production app is now down and I can't switch back to the prior auth mode:

Getting this error:: "errorCode" : "OAU-251", "message" : "Unauthorized for this grant type"

I tested all of this on a separate sandbox account successfully and then switched our existing live app to use JWT and now it doesn't work.