So you are ready to develop a SharePoint app and have followed this MSDN article to get everything set up. F5 in visual studio, clicked Trust It to "Do you trust <name of your app>?" from SharePoint, and then you are getting
The remote server returned an error: (401) Unauthorized
Chances are the IssuerId in provider-hosted application web.config is listed as a UPPER CASE Guid string.
<add key="IssuerId" value="983802AC-C7A9-4B50-8100-2C6E2773B9CF" />
To resolve above issue, all you needed to do is to make it a lowercase Guid string
<add key="IssuerId" value="983802ac-c7a9-4b50-8100-2c6e2773b9cf" />
Also, make sure the IssuerID in web.config match the one you see from Get-SPTrustedSecurityTokenIssuer SharePoint Powershell command. (If you don't see anything listed from this command, go back to steps in MSDN article)
I hope this helps