JavaScript have error fetch inside else fetch

Linda

I get an error, when I am using one method onLogin with the fetch inside the other onRegister. I don't fully understand where the mistake. Separately, the methods are working fine.

Stack:

Create React App React router v6

Access to fetch at 'https://auth.blablabla/signin' from origin 'http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

function onRegister({ email, password }) {
    auth.register(email, password)
      .then((res) => {
        try {
          // here when when using the method I get the error.
          onLogin(email, password);
        } catch (error) {
          console.log(error);
        }
      }).catch(err => {
        console.log('Error to registartion: ', err);
      });
  }

  function onLogin({ email, password }) {
    auth.login(email, password)
      .then(res => {
        if (res.token) {
          setIsLoggedIn(true);
          localStorage.setItem('jwt', res.token);
          setEmail(email);
          navigate('/');
        }
      }).catch(err => {
        console.log('Error log: ', err);
      }
      );
  }

But this is methods for auth: register and login

export const register = (email, password) => {
    return fetch(`${BASE_URL}/signup`, {
        method: 'POST',
        headers: {
            "Content-Type": "application/json",
            'Accept': 'application/json'
        },
        body: JSON.stringify({
            password,
            email
        })
    })
    .then((res) => {
        try {
            if (res.status === 201) {
                return res.json();
            }
            if (res === 400) {
                return Promise.reject('in one of the fields of the error');
            }
        } catch (error) {
            console.log(error);
        }
    })
    .then((res) => {
        return res;
    })
        .catch((error) => console.log(error))
}

export const login = (email, password) => {
    return fetch(`${BASE_URL}/signin`, {
        method: 'POST',
        headers: {
            "Content-Type": "application/json",
            'Accept': 'application/json'
        },
        body: JSON.stringify({
            password,
            email
        })
    })
    .then((res) => {
        try {
            if (res.ok) {
                return res.json();
            }
            if (res.status === 401) {
                return Promise.reject('user for this email not found');
            }
            if (res.status === 400) {
                return Promise.reject('fill in all the fields');
            }
        } catch (error) {
            console.log(error);
        }
        
    })
    .then((data) => {
        localStorage.setItem('jwt', data.token)
        localStorage.setItem('email', email);
        return data;
    })
    .catch((err) => console.log(err))
}