multisite user errors

Im trying to add new people from my team onto my site but when i put in a username and email they get sent a link that looks like /wp-activate.php? and the page does not work

on the page it says ‘an error has occurred during the activation could not create user’

I made my site multisite and this is happening i need my site to be multisite but i cant do anything when users links wont work im unsure how to find the cause of the issue or how to fix it

Im tried skipping the link but when i was trying to edit the themes php files it said it could not be edited and the changes were reverted