POST in API don't run, error message 500

Hello,

I have a lesson with javascript for beginner, and i have an issue with the API "post " function. I don’t understand why my code doesn’t run. The aim is to post a new work (new image with title and catégory) or is to publish changement. There is my javascript code. If you see an error, please tell me. Thank a lot. here is my repo github :

branch main

And here is my code :

/* Send the image to server */
let image;
const submitButton = document.querySelector("#submit-button");
const titleInput = document.querySelector("#title");
const categorySelect = document.querySelector("#category");
const background = document.querySelector(".background");
const img = document.createElement("img");


submitButton.addEventListener("click", function(event) {
    event.preventDefault();
    const title = titleInput.value;
    const category = categorySelect.value;
    console.log(image);
    if (!title || !category) {
        console.error("les champs sont tous requis");
        return;
    }
    
    if (!image) {
        console.error("Une image est obligatoire");
    }
    const formData = new FormData();
    formData.append("image", image, image.name);
    formData.append("title", title);
    formData.append("category", category);

    try {
        const response = fetch("http://localhost:5678/api/works", {
            method: "POST",
            headers: {
                "Content-Type": "application/json",
                "Authorization": `Bearer ${sessionStorage.token}`
            },
            body: formData
        });
        
        submitButton.style.backgroundColor = "#1D6154";
    }
    catch (error) {
        console.error(error);
        submitButton.style.backgroundColor = "#A7A7A7";
    }
});

/* upload an image */
let imgSrc;
document.querySelector(".add-photo").addEventListener("click", function() {
    const input = document.createElement("input");
    input.type = "file";
    input.accept = "image/jpg, image/png";
    input.click();
/* Check the file type and size */
    input.addEventListener("change", async function() {
        image = input.files[0];
        if (image.type !== "image/jpg" && image.type !== "image/png") {
            document.getElementById("errorMessage").innerHTML = "jpg ou png obligatoire";
            return;
        }
        if (image.size > 4 * 1024 *1024) {
            document.getElementById("errorMessage").innerHTML = "4mo maximum";
            return;
        }
        /* Replace by image loaded, the default message jpg png 4mo max */
        background.innerHTML = "";
        const reader = new FileReader();
        reader.readAsDataURL(image);
        reader.onload = function() {
            img.src = reader.result;
            imgSrc = reader.result;
            img.style.width = "30%";
            background.appendChild(img);
            if (titleInput.value && categorySelect.value && img.src) {
                submitButton.disabled = false;
            }
            else {
                submitButton.disabled = true;
            }
        }
    });
});

/* take categories from API in loading new image*/
async function getCategories() {
    const response = await fetch("http://localhost:5678/api/categories");
    const categories = await response.json();
    const select = document.querySelector("#category");
    for (let i = 0; i < categories.length; i++) {
        let option = document.createElement("option");
        option.value = categories[i].id;
        option.innerHTML = categories[i].name;
        select.appendChild(option);
    }
}
getCategories();

/* function submit button valider */
function validate() {
    if (titleInput.value && categorySelect.value && img.src) {
        submitButton.disabled = false;
    }
    else {
        submitButton.disabled = true;
    }
}
titleInput.addEventListener("input", validate);
categorySelect.addEventListener("input", validate);
validate();

/* publish all work on index.html */
const publish = document.querySelector(".publish");

publish.addEventListener("click", function() {
    console.log("publish");
    const portfolio = document.getElementById("portfolio");
    portfolio.innerHTML = "";
    fetch("http://localhost:5678/api/works", {
        method: "POST",
        headers: {
            "Content-Type": "application/json",
            "Authorization": `Bearer ${sessionStorage.token}`
        },  
        body: JSON.stringify({
            title: titleInput.value,
            category: categorySelect.value,
            image: imgSrc
        })
    })  
    .then(function (response) {
        return response.json();
    })
    .then(function (work) {
        displayPortfolio(work);
    })
    .catch(function (error) {
        console.error(error);
    });
});