如HTTP Callable Cloud Functions文档中所述：
若要在异步操作后返回数据，请返回promise。
下面是一个例子：

const sanitizedMessage = sanitizer.sanitizeText(text); // Sanitize the message.
return admin.database().ref('/messages').push({
  text: sanitizedMessage,
  author: { uid, name, picture, email },
}).then(() => {
  console.log('New Message written');
  // Returning the sanitized message to the client.
  return { text: sanitizedMessage };
})

所以你需要调整你的代码如下：

exports.sendMessage = functions.https.onCall((data, context) => {
  if (context.auth.uid == undefined) {
    console.warn("SEND MESSAGE: USER NOT SIGNED IN");
    //Here send back an error as explained here: https://firebase.google.com/docs/functions/callable#handle_errors
  }

  console.log("Sending message:", data)
  const matchId = data["matchId"];
  const message = data["message"]
  const uid = context.auth.uid

  //Note the return on next line
  return admin.firestore().collection(MatchingUsers).doc(matchId).collection(UserMessages).add({
    type: "text",
    from: uid,
    message: message,
    timestamp: admin.firestore.Timestamp.now()
  }).then(result => {
    console.log("Message sent");
    return { text: "Message sent" };
  }).catch(err => {
    console.log("Error sending mesage:", err);
    //Here, again, send back an error as explained here: https://firebase.google.com/docs/functions/callable#handle_errors
  })
})

如果您不想向客户端返回值，您可以如下操作，当add()异步方法返回的Promise解析时返回null。（没有测试，但它应该工作）。

exports.sendMessage = functions.https.onCall((data, context) => {
  if (context.auth.uid == undefined) {
    console.warn("SEND MESSAGE: USER NOT SIGNED IN");
    return null;
  }

  console.log("Sending message:", data)
  const matchId = data["matchId"];
  const message = data["message"]
  const uid = context.auth.uid

  //Note the return on next line
  return admin.firestore().collection(MatchingUsers).doc(matchId).collection(UserMessages).add({
    type: "text",
    from: uid,
    message: message,
    timestamp: admin.firestore.Timestamp.now()
  }).then(result => {
    console.log("Message sent");   //Actually, if you don't need this console.log() you can remove this entire then() block, returning the promise from add() is enough
    return null;
  }).catch(err => {
    console.log("Error sending mesage:", err);
    return null;
  })
})