The configuration file 'appsettings.json' was not found and is not optional

In My case the file appsettings.json existed in project folder, but it was set to Do not copy, I changed the setting to Copy always (see images below). And it worked for me.

It will automatically added following XML to your project.csproj file:

<ItemGroup>
    <Content Update="appsettings.json">
      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
    </Content>
</ItemGroup>

I have looked at other answer, project.json is dead as this answer says.

In later .net core versions a *.csproj file is used instead of the project.json file.

You can modify the file to get the desired result by adding:

   <ItemGroup>
      <Content Update="appsettings.json">
        <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
      </Content>
   </ItemGroup>

For me the error was using Directory.GetCurrentDirectory(). This worked fine running locally but on a production server it failed when the program was started from Powershell. Replaced with Assembly.GetEntryAssembly().Location and everything worked.

Complete code:

var builder = new ConfigurationBuilder()
        .SetBasePath(Path.GetDirectoryName(Assembly.GetEntryAssembly().Location))
        .AddJsonFile("appsettings.json");

var configuration = builder.Build();