POST request to Sanity CMS using Next.js API & next-connect resulting in 500 ERROR

Question

I am having trouble making a POST request of user info on a register form to the Sanity CMS.I am using Next.js & next-connect. I am getting a 500 error when making the request.

Here is a snippet of code from the register.js front end page:

const submitHandler = async ({
    firstName,
    lastName,
    email,
    password,
    confirmPassword,
  }) => {
    if (password !== confirmPassword) {
      enqueueSnackbar("Passwords don't match", { variant: "error" });
      return;
    }
    try {
      const { data } = await axios.post(`/api/users/register`, {
        firstName,
        lastName,
        email,
        password,
      });
      console.log("hit success")
      dispatch({ type: "USER_LOGIN", payload: data });
      jsCookie.set("userInfo", JSON.stringify(data));
      router.push("/");
    } catch (err) {
      console.log(err.message)
      enqueueSnackbar("Something went wrong, please try again.", { variant: "error" });
    }
  };

Here is the code from the api/users/router.js file:

handler.post(async (req, res) => {
  console.log("hit handler")
  const projectId = client.projectId;
  const dataset = client.dataset;
  const apiVersion = client.apiVersion;
  const tokenWithWriteAccess = process.env.SANITY_AUTH_TOKEN;
  const createMutations = [
    {
      create: {
        _type: "user",
        firstName: req.body.firstName,
        lastName: req.body.lastName,
        email: req.body.email,
        password: bcrypt.hashSync(req.body.password),
        isAdmin: false,
        isWholesale: false,
      },
    },
  ];
  const { data } = await axios.post(
    `https://${projectId}.api.sanity.io/v${apiVersion}/data/mutate/${dataset}`,
    {
      mutations: createMutations,
    },
    {
      headers: {
        "Content-type": "application/json",
        Authorization: `Bearer ${tokenWithWriteAccess}`,
      },
    }
  );
  const userId = data.results[0].id;
  const user = {
    _id: userId,
    firstName: req.body.firstName,
    lastName: req.body.lastName,
    email: req.body.email,
    isWholesale: false,
    isAdmin: false,
  };
   const token = signToken(user);
  res.send({ ...user, token });
});

• I've done my best to double check the format of the GROQ requests.
• I made sure that I have localhost allowed through CORS.
• I have double checked that I am using a current access token and all other variables such as projectID, etc. are correct.
• I've verified that the my form handlers are working correctly and that the object I am sending matches the schema I set up in Sanity.io.
• I have used Postman to make a successful GET request to the api/users/register.js end point.

Thanks in advance for any assistance!