You are trying to change the username attribute for which the user does not have permissions.

This can be an issue with the security setup, look at what roles a user can manage or you'll need to implement this with a different construction in your microflow.

For an example of a sign-up functionality have a look at the marketplace module: Forgot password, there is a signup functionality available in that module.

Go to the entity that have the username attribute, then double click on it then go to the access rules tab, double click on anonymous user role and give the username attribute read and write