If you are getting this error message with Web API in Blazor WeAssembly application. You can troubleshoot following ways.

1. Take the api end point and trigger with PostMan
2. If it is not triggering, then try to see the api method attribute. So many we missed out input parameter of Web api method.

In my case, I was missed out InstanceId for Web API Method

 [HttpGet]
 [Route("[action]]
 public async Task<List<ProductModel>> GetProducts(int instanceId) => await _service.GetProducts(instanceId);

Fix: Pass the instanceId parameter to web api method.

 [HttpGet]
 [Route("[action]/{instanceId}")]
 public async Task<List<ProductModel>> GetProducts(int instanceId) => await _service.GetProducts(instanceId);