All error codes

server_not_found

HTTP 404 Server not found

What happened

The server id is not a server of this project. It does not exist, it was deleted, or it belongs to another project - all three answer the same way, so this response is not evidence that the id is unused.

What to do

List the project's servers with GET /v1/servers and use an id from there. A server you have just created is addressable at once - the creation response carries its id and a Location header, and reading it while the task is still running returns the server with status creating.

Retrying

Repeating this request will not help.

How it arrives

{
    "type": "https://docs.fiberax.com/errors/server_not_found",
    "title": "Server not found",
    "status": 404,
    "code": "server_not_found",
    "request_id": "a277dcd1ca652c65"
}

Branch your code on code, never on the wording of title.