Erreur lorsque je fais >hello ( prefix = > pour info )

(node:26028) UnhandledPromiseRejectionWarning: ReferenceError: prefix is not defined
    at Object.module.exports.run (c:\Users\Quentin\Desktop\Botest\commands\hello.js:4:28)
    at Client.<anonymous> (c:\Users\Quentin\Desktop\Botest\index.js:38:27)
    at Client.emit (events.js:210:5)
    at MessageCreateHandler.handle (c:\Users\Quentin\Desktop\Botest\node_modules\discord.js\src\client\websocket\packets\handlers\MessageCreate.js:9:34)
    at WebSocketPacketManager.handle (c:\Users\Quentin\Desktop\Botest\node_modules\discord.js\src\client\websocket\packets\WebSocketPacketManager.js:105:65)
    at WebSocketConnection.onPacket (c:\Users\Quentin\Desktop\Botest\node_modules\discord.js\src\client\websocket\WebSocketConnection.js:333:35)
    at WebSocketConnection.onMessage (c:\Users\Quentin\Desktop\Botest\node_modules\discord.js\src\client\websocket\WebSocketConnection.js:296:17)
    at WebSocket.onMessage (c:\Users\Quentin\Desktop\Botest\node_modules\ws\lib\event-target.js:120:16)
    at WebSocket.emit (events.js:210:5)
    at Receiver.receiverOnMessage (c:\Users\Quentin\Desktop\Botest\node_modules\ws\lib\websocket.js:789:20)
warning.js:27
(node:26028) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
warning.js:27
(node:26028) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.