Azure Active Directory B2C Customization of signup and signin custom policy

Question

We have initially used Sign-up Sign-in policy, but our customer required Sign-up page to be shown as default and have a login button to take to the Sign in page.

We were able to achieve showing Sign-up page as default using custom policy but we are getting page not found (attached) when we click on "Login" button.

Any pointers in fixing this is highly appreciated.