I’m not sure what the structure of textbox.value is but you need to stringify your data before sending it like this:

headers = {
    'Content-type':'application/json',
    'Accept':'application/json'
}

function send(){
    const line = textbox.value;
    $.ajax({
        type: "POST",
        url: "/send",
        data: JSON.stringify(line),
        headers: headers,
    });
}

And in your Flask code, you are already using request.get_json() to parse the JSON data so you need to remove json.load() since you are trying to parse data from a POST request and not from a file. Your Flask code should look like this:

@app.route("/send", methods=["POST"])
def print_data():
    data = request.get_json().get("data")
    main.FileMan.post_data(str(data))
}