The most common HTTP error responses as follow:

Error responseDescription
400 Bad RequestThe server cannot or will not process the request due to something that is perceived to be a client error (e.g., malformed request syntax, invalid request message framing, or deceptive request routing).
401 UnauthorizedThe client must authenticate itself to get the requested response.
403 ForbiddenThe client does not have access rights to the content; The server is refusing to give the requested resource.
404 Not FoundThe server cannot find the requested resource. In the browser, this means the URL is not recognized. In an API, this can also mean that the endpoint is valid but the resource itself does not exist.
405 Method Not AllowedThe request method is known by the server but is not supported by the target resource. For example, an API may not allow calling DELETE to remove a resource.
408 Request TimeoutThe requested server has closed the connection due to a timeout, whatever the value is set at. Try sending the request again. If the issue persists, try changing the endpoint or session type for your request.
500 Internal Server ErrorThe server has encountered an internal error.
502 Bad GatewayThis error response means that the server, while working as a gateway to get a response needed to handle the request, got an invalid response.
503 Service UnavailableThe server is not ready to handle the request. Common causes are a server that is down for maintenance or that is overloaded.
504 Gateway TimeoutThis error response is given when the server is acting as a gateway and cannot get a response in time.
Proxy Error Code