Hello Gleb,
Hope you're doing well.
That happens because you're trying to access to a Screen that has probably the Registered role.
By default, when you create a Screen in OutSystems, it has only the Registered role ticket:

When you tick the Registered role only, this means that only registered users (that can execute a login in the OutSystems platform) can access to your screen. If you are not logged in, the application (also by default) returns you to a Login page (this page is already implemented by default in the module - check Common UI Flow).
If you tick the Anonymous role, this means that everyone will be able to access that page. It is just necessary to know the URL.
My suggestion is: if this is only for testing purposes, you can tick that Anonymous role in your screens and you won't need to specify credentials.
If this is not the case, you will need to:
- Go to this page: https://<your-environment>/Users
- Create a user in this Users application
- Enter with those credentials
This Users application provides users for access to your application because your module has defined Users application as the User Provider. You can also check this in the properties of the module:

Please read more in this documentation:
https://success.outsystems.com/Documentation/11/Developing_an_Application/Secure_the_Application/User_Roles
https://success.outsystems.com/Documentation/11/Developing_an_Application/Secure_the_Application/End_User_Management
Hope that this helps you!
Kind regards,
Rui Barradas