Nuget connection attempt failed "Unable to load the service index for source"

Deleting %AppData%\Roaming\NuGet\NuGet.Config and restarting VS2019 worked for me.

Similar to https://github.com/NuGet/Home/issues/3281

回答2

I was getting the same error while trying to browse the NuGet Package, to resolve the same followed below step:

1- go to %appdata%\NuGet\NuGet.config

2- Verify the urls mentioned in that config

3- Remove the url which is not required

4- Restart visual studio and check

回答3

A developer of the nuget-package manager suggested in 2019 to disable tls 1.3 as a workaround (see issue 7705).

Open Registry Editor by pressing Win + R and type regedit Enter

Navigate to:

\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.3\Client

Change the value of DisabledByDefault key 0 to 1

Then restart the Visual Studio.

Read more about TLS on wikipedia
Read more about issue 7705 w.r.t. NuGet at github

3rd party edit

Be aware that this disables tls 1.3 for the os not just for nuget or dotnet.
Windows 10 version 1909 did contain an experimental implementation of TLS 1.3 but later versions should be fine. Based on one answer from Unable to browse nuget packages you can test if tls is the problem via a console programm

static async Task Main(string[] args)
{
    var client = new HttpClient();
    string uri = "https://apiint.nugettest.org/v3-index/index.json";
    var response = await client.GetAsync(uri);
    string msg = "If you see this, your machine has no TLS/SSL issues with nuget.org";
    Console.WriteLine(msg);

}

StatusCode: 401, ReasonPhrase: 'Unauthorized', Version: 1.1, Content: System.Net.Http.StreamContent, Headers:
{
  Pragma: no-cache
  Cache-Control: no-cache
  P3P: CP="CAO DSP COR ADMa DEV CONo TELo CUR PSA PSD TAI IVDo OUR SAMi BUS DEM NAV STA UNI COM INT PHY ONL FIN PUR LOC CNT"
  WWW-Authenticate: Bearer authorization_uri=https://login.windows.net/4c1d9e0f-5c27-4228-a35a-de7b4083ff7b
  WWW-Authenticate: Basic realm="https://pkgsprodsea1.pkgs.visualstudio.com/"
  WWW-Authenticate: TFS-Federated
  X-TFS-ProcessId: 6fc8918c-f900-4df8-964a-f30dc645a292
  Strict-Transport-Security: max-age=31536000; includeSubDomains
  ActivityId: 03bc0847-be61-42c4-b77e-29887273a4a5
  X-TFS-Session: 03bc0847-be61-42c4-b77e-29887273a4a5
  X-VSS-E2EID: 03bc0847-be61-42c4-b77e-29887273a4a5
  X-VSS-SenderDeploymentId: c2de059f-04a4-1d6b-3b8b-c264075f8958
  X-FRAME-OPTIONS: SAMEORIGIN
  X-TFS-FedAuthRealm: https://pkgsprodsea1.pkgs.visualstudio.com/
  X-TFS-FedAuthIssuer: https://www.visualstudio.com/
  X-VSS-AuthorizationEndpoint: https://vssps.dev.azure.com/edenred-itec-rdc/
  X-VSS-ResourceTenant: 4c1d9e0f-5c27-4228-a35a-de7b4083ff7b
  X-VSS-S2STargetService: 00000030-0000-8888-8000-000000000000/visualstudio.com
  X-TFS-FedAuthRedirect: https://spsprodea1.vssps.visualstudio.com/_signin?realm=pkgs.dev.azure.com&reply_to=https%3A%2F%2Fpkgs.dev.azure.com%2Fedenred-itec-rdc%2F_packaging%2FEdenred.Rdc%2Fnuget%2Fv3%2Findex.json&redirect=1&hid=49cf9e98-b607-4f1d-9453-1b716e1de413&context=eyJodCI6MiwiaGlkIjoiMGZlMjIyNjAtYjkwZS00ZDVhLTk1N2ItZmM4MGNkOWE0NThhIiwicXMiOnt9LCJyciI6IiIsInZoIjoiIiwiY3YiOiIiLCJjcyI6IiJ90#ctx=eyJTaWduSW5Db29raWVEb21haW5zIjpbImh0dHBzOi8vbG9naW4ud2luZG93cy5uZXQiXX01
  Request-Context: appId=cid-v1:44bb2f7d-18d2-472b-b6db-4777a144596c
  Access-Control-Expose-Headers: Request-Context
  X-Content-Type-Options: nosniff
  X-Cache: CONFIG_NOCACHE
  X-MSEdge-Ref: Ref A: 77AECA4794C044CDB2EFA45D9AF82B66 Ref B: TYBEDGE0520 Ref C: 2022-09-27T03:38:26Z
  Date: Tue, 27 Sep 2022 03:38:27 GMT
  Content-Length: 343
  Content-Type: application/json; charset=utf-8
  Expires: -1
}