You need to be CORS compatible. Sounds like you EBS Is not behaving as it’s not supporting CORS.
If you have having issues with the example. The best thing to do is skip the example and just get building. Since the problem you are describing is not a simple one to fix as it could be literally anything.
If you are 404’ing this could mean the path/route to the EBS isn’t correct or it’s just not started or theres something else going on