"The project 'Web' must provide a value for Configuration" error after migrating to .NET Core 3

The issue turned out to be that I was still referencing Microsoft.AspNetCore.Razor.Design Version="2.2.0" in the .proj file's package references. Deleting that reference (which isn't needed at all as Razor.Design is now part of AspNetCore library) fixed the issue.

Once I'd done that, I then got hundreds of errors about nullable objects being a new feature not compatible with razor. That was because I had <LangVersion>Latest</LangVersion> in my .proj file. Removing that line fixed that issue and got the project running again.

(In some cases you might need to clean and rebuild and/or restart VS too, according to comments on the github thread)

If that doesn't solve it, it's possible that one the NuGet packages used by your project is the cause. Try removing the dependencies to see if that clears the issue, and then re-add them one at a time to work out which NuGet package is the cause.

I've had the same issue and it was solved by removing the following references:

"Microsoft.AspNetCore.Mvc" Version="2.2.0"

"Microsoft.AspNetCore.Mvc.Razor.ViewCompilation" Version="2.2.0"

There is an MS document title "Migrate from ASP.NET Core 2.2 to 3.0". Under "Update the project file", it states,

"A large number of NuGet packages aren't produced for ASP.NET Core 3.0. Such package references should be removed from your project file"

The two given as an example are:

• Microsoft.AspNetCore.App
• Microsoft.AspNetCore.Razor.Design

Below that, you can expand to see all the packages that are no longer being produced:

• Microsoft.AspNetCore
• Microsoft.AspNetCore.All
• Microsoft.AspNetCore.App
• Microsoft.AspNetCore.Antiforgery
• Microsoft.AspNetCore.Authentication
• Microsoft.AspNetCore.Authentication.Abstractions
• Microsoft.AspNetCore.Authentication.Cookies
• Microsoft.AspNetCore.Authentication.Core
• Microsoft.AspNetCore.Authentication.OAuth
• Microsoft.AspNetCore.Authorization.Policy
• Microsoft.AspNetCore.CookiePolicy
• Microsoft.AspNetCore.Cors
• Microsoft.AspNetCore.Diagnostics
• Microsoft.AspNetCore.Diagnostics.HealthChecks
• Microsoft.AspNetCore.HostFiltering
• Microsoft.AspNetCore.Hosting
• Microsoft.AspNetCore.Hosting.Abstractions
• Microsoft.AspNetCore.Hosting.Server.Abstractions
• Microsoft.AspNetCore.Http
• Microsoft.AspNetCore.Http.Abstractions
• Microsoft.AspNetCore.Http.Connections
• Microsoft.AspNetCore.Http.Extensions
• Microsoft.AspNetCore.HttpOverrides
• Microsoft.AspNetCore.HttpsPolicy
• Microsoft.AspNetCore.Identity
• Microsoft.AspNetCore.Localization
• Microsoft.AspNetCore.Localization.Routing
• Microsoft.AspNetCore.Mvc
• Microsoft.AspNetCore.Mvc.Abstractions
• Microsoft.AspNetCore.Mvc.Analyzers
• Microsoft.AspNetCore.Mvc.ApiExplorer
• Microsoft.AspNetCore.Mvc.Api.Analyzers
• Microsoft.AspNetCore.Mvc.Core
• Microsoft.AspNetCore.Mvc.Cors
• Microsoft.AspNetCore.Mvc.DataAnnotations
• Microsoft.AspNetCore.Mvc.Formatters.Json
• Microsoft.AspNetCore.Mvc.Formatters.Xml
• Microsoft.AspNetCore.Mvc.Localization
• Microsoft.AspNetCore.Mvc.Razor
• Microsoft.AspNetCore.Mvc.Razor.ViewCompilation
• Microsoft.AspNetCore.Mvc.RazorPages
• Microsoft.AspNetCore.Mvc.TagHelpers
• Microsoft.AspNetCore.Mvc.ViewFeatures
• Microsoft.AspNetCore.Razor
• Microsoft.AspNetCore.Razor.Runtime
• Microsoft.AspNetCore.Razor.Design
• Microsoft.AspNetCore.ResponseCaching
• Microsoft.AspNetCore.ResponseCaching.Abstractions
• Microsoft.AspNetCore.ResponseCompression
• Microsoft.AspNetCore.Rewrite
• Microsoft.AspNetCore.Routing
• Microsoft.AspNetCore.Routing.Abstractions
• Microsoft.AspNetCore.Server.HttpSys
• Microsoft.AspNetCore.Server.IIS
• Microsoft.AspNetCore.Server.IISIntegration
• Microsoft.AspNetCore.Server.Kestrel
• Microsoft.AspNetCore.Server.Kestrel.Core
• Microsoft.AspNetCore.Server.Kestrel.Https
• Microsoft.AspNetCore.Server.Kestrel.Transport.Abstractions
• Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets
• Microsoft.AspNetCore.Session
• Microsoft.AspNetCore.SignalR
• Microsoft.AspNetCore.SignalR.Core
• Microsoft.AspNetCore.StaticFiles
• Microsoft.AspNetCore.WebSockets
• Microsoft.AspNetCore.WebUtilities
• Microsoft.Net.Http.Headers