Have you setup your own custom lightboxes for login and signup and setup the member signup for your own custom lightbox as shown here.
https://support.wix.com/en/article/corvid-enabling-custom-site-registration
Plus, have you got any code on your page that is something like the members profile page like as shown here.
https://support.wix.com/en/article/corvid-tutorial-building-your-own-members-area
Basically, have you got any code in your login setup that is like this.
wixUsers.promptLogin( {"mode": "login"} )
Well, if you have, then you need to change the mode option to signup instead, see the API reference for more. https://www.wix.com/corvid/reference/wix-users.html#promptLogin
So it should say this instead.
wixUsers.promptLogin( {"mode": "signup"} )