No UseDatabaseErrorPage() extension method in Net Core 3.0
Add a reference to 'Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore', it appears that function is in that package now.
use
Install-Package Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore -Version 3.1.5
[CLI]
dotnet add package Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore --version 3.1.2
[StartUp.cs]
using Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore;