Problem: The type or namespace UI does not exist in the namespace Microsoft.AspNetCore.Identity bootstrap dotnet core 3.0
Solution: If you had upgraded Microsoft.AspNetCore.Identity.UI version="3.0.0" Nuget Package to version="3.1.0", revert the changes. Simply right click on your project name and navigate to "Edit Project File" and find the version number version="3.1.0" and change it to version="3.0.0". Build your project and you will be good to go.
Thanks man, it really helped.