Hey Snehal Can you share the Screen shot of what is the error throwing while logging using Demo users i believe you are logging using the id as shown below (example)
By clicking on the edit demo user you will find username and PWD you can copy pwd by clicking on copy password to clipboard.
I think demo user will help to switch between different User Roles when you sign in .
You need to sign in first with “Administrator” User Role. You can Use same credential for Hybrid App which you use to sign in your web app. And then you switch between Demo User Role.