The axios post is posting using the wrong form type.
The documentation specifies data-urlencode, and you/axios are probably sending “multipart/form-data” (instead of “application/x-www-form-urlencoded”) in error causing the problem
So try
axios.post('https://id.twitch.tv/oauth2/token?grant_type=refresh_token&refresh_token=' + encodeURIComponent("RefreshTokenDeliveredFromGamesCallLandsHere") + '&client_id=mYcLiEnTiD&client_secret=sEcReThErE', {},
instead.