这是因为你同时使用了异步和回调函数。因为回调函数和异步函数并行执行，所以导致了这个问题。
最好的方法做较少的变化，如下所示，放在else条件。

exports.create = async (req, res) => { 

    // Check if the attendee already exists
    let user = await Attendee.findOne({ email: req.body.email });
    if (user) {
      return res
        .status(400)
        .json({ error: "An attendee with this email already exists" });
    }else {

        // sending verification link to the user
        const mailData = {
          from: process.env.USER_ID,
          
          to: req.body.email,
          
          subject: "Email Verification link",
          text: `Click on the link and verify your email`,
          // verification link 
          html: `<a href="localhost:8000/api/verify">Click here to verify</a>`,
        };
    
        // sending the email
        transporter.sendMail(mailData, (error, info) => {
          if (error) console.log(error);
          console.log("Email Sent Successfully");
          
        });
        res.status(200).json({message:"Verification link has been sent to your email. Please verify your email"})
    }
    
};

res.send(STATUS_CODE).json()

已弃用。有关详细信息，请参阅此处。
请改为使用以下命令：

res.status(200).json({message:"Verification link has been sent to your email. Please verify your email"}})

(Also见此）