FYI that error can happen even with a working app. We see it in our prod logs and it is something about how .NET Core's SPA middleware is working. Unless it is causing you actual issues I wouldn't worry about it. We haven't taken the time to figure it out yet because our app is still working fine and it has been this way for months.
As for accessing the API I'm a little confused. You said you built an API app with React but React is a UI front end. APIs don't have front ends. I'm assuming this is a web app that uses React for the front end and .NET Core/6 for the backend. When you attempt to browse to the site you should get a response from the server with the relevant status code and message. What are you getting?
I'm assuming you configured IIS properly for .NET Core as documented here. Specifically you installed the hosting bundle on your server, you set up the app pool correctly, permissions are correct and your app deployed fully.