Checking if username and/or email are in use javaScript

I am creating a webpage for a school proyect and I am having issues with the register tab. A friend is coding the backend on pgAdmin and its running on a Vercel server. The issue is that every time a new user register it says that the email and username are in use when they are not. This is my FunctionsR.js:

var isPassOk;
var isUsernameOk;
var isEmailOk;

function showPass() {
var x = document.getElementById("passwordId");
var y = document.getElementById("checkPasswordId");
var i = document.getElementById("passIconId");
if (x.type === "password") {
    x.type = "text";
    y.type = "text";
    i.classList.remove('bx-show');
    i.classList.add('bx-show');
} else {
    x.type = "password";
    y.type = "password";
    i.classList.remove('bx-show');
    i.classList.add('bx-hide');
}
}

function checkMail() {
var email = document.getElementById("emailId").value;
var alert = document.querySelector(".alertEmail");
var alert2 = document.querySelector(".alertEmail2");

var emailPattern = /^[a-zA-Z0-9._-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,6}$/;
if (!emailPattern.test(email)) {
    alert.style.display = 'block';
    isEmailOk = false;
    return isEmailOk;
}

emailInUse(email).then(errorMessage => {
    if (errorMessage) {
        alert2.style.display = 'block';
        isEmailOk = false;
    } else {
        isEmailOk = true;
    }
    return isEmailOk;
}).catch(error => {
    console.error('Error:', error);
    isEmailOk = false;
    return isEmailOk;
});
}

function emailInUse(email) {
return fetch('https://centinel-ai.vercel.app/api/register', {
    method: 'POST',
    headers: {
        'Content-Type': 'application/json'
    },
    body: JSON.stringify({ email })
})
.then(response => response.json())
.then(data => {
    if (data.error && data.error === 'Email already in use') {
        return 'Email already in use'; 
    }
    return null; 
})
.catch(error => {
    console.error('Error:', error);
    return 'An error occurred';
});
}

function checkUsername() {
var username = document.getElementById("usernameId").value;
var alert = document.querySelector(".alertUsername");
var alert2 = document.querySelector(".alertUsername2");

if (username.length > 25 || username.length < 4) {
    alert.style.display = 'block';
    isUsernameOk = false;
}
usernameInUse(username).then(errorMessage => {
    if (errorMessage) {
        alert2.style.display = 'block';
        isUsernameOk = false;
    } else {
        isUsernameOk = true;
    }
    return isUsernameOk;
}).catch(error => {
    console.error('Error:', error);
    isUsernameOk = false;
    return isUsernameOk;
});
}

function usernameInUse(username) {
return fetch('https://centinel-ai.vercel.app/api/register', {
    method: 'POST',
    headers: {
        'Content-Type': 'application/json'
    },
    body: JSON.stringify({ username })
})
.then(response => response.text())
.then(text => {
    try {
        const data = JSON.parse(text);
        if (data.error && data.error === 'Username already in use') {
            return 'Username already in use'; 
        }
        return null; 
    } catch (error) {
        console.error('Error parsing JSON:', error);
        return 'An error occurred';
    }
})
.catch(error => {
    console.error('Error:', error);
    return 'An error occurred';
});   
}

function checkPass() {
var password = document.getElementById("passwordId").value;
var confirmPassword = document.getElementById("checkPasswordId").value;
var errorMessage = document.getElementById("error-message");

if (password !== confirmPassword) {
    errorMessage.style.display = 'block';
    document.getElementById("passwordId").style.color = 'red';
    document.getElementById("checkPasswordId").style.color = 'red';
    isPassOk = false;
} else {
    errorMessage.style.display = 'none';
    document.getElementById("passwordId").style.color = 'white';
    document.getElementById("checkPasswordId").style.color = 'white';
    isPassOk = true;
}
return isPassOk;
}

function registerUser() {

var username = document.getElementById('usernameId').value;
var email = document.getElementById('emailId').value;
var password = document.getElementById('passwordId').value;

if(isPassOk && isUsernameOk && isEmailOk){
    fetch('https://centinel-ai.vercel.app/api/register', {
        method: 'POST',
        headers: {
            'Content-Type': 'application/json'
        },
        body: JSON.stringify({ email, username, password })
    })
        .then(response => response.text())
        .then(message => console.log(message))
        .catch(error => {
            console.error('Error:', error);
            console.error('Status:', error.status);
            console.error('Status Text:', error.statusText);
        });   
}   
}

function onRegisterSubmit(e){
console.log("Submited")
e.preventDefault();

checkPass();
checkMail();
checkUsername();

registerUser();
}

document.addEventListener("DOMContentLoaded", () => {
const registerForm = document.getElementById("register");

registerForm.addEventListener("submit", onRegisterSubmit);
})

I am new to webpage development so I depend on using AI a lot. I can understand almost everything but I can’t troubleshoot very well. Everything helps. Thank you!