@laveesingh is this happening in a particular browser? What version of the Auth0.js library are you using? Can you try specifying the popupOrigin and make sure the redirectUri and the popupOrigin both match the website origin (full domain)?
Using the popup for authorization can sometimes cause issues in some browsers (and different versions of the same browser) depending on how popups are handled. Popups must be explicitly enabled in some browsers, such as Chrome I believe, so we typically recommend using a redirect mode.