I was working on upgrading from Sitecore 9.1 to version 10.3.1 and encountered a problem where the Identity Server where it failed to load.

Issue

When accessing the Identity Server, an error with code 502.5 was encountered. The Identity Server, being a .NET Core application, fails to start.

HTTP Error 502.5 - ANCM Out-Of-Process Startup Failure

Debug

Ensure the correct version of .NET is installed on the server.
Open a command prompt and execute to confirm the version is 6.0

dotnet --info

Navigate to the Identity server installation folder and execute

dotnet Sitecore.IdentityServer.Host.dll

Fix

Looking at the error and found that the license file was missing. Once it was copied over to the appropriate location it started working as expected.