Headers already sent error PHP form

<div class="form-container upload">
        <form class="default-form upload" action="upload-game" method="post" enctype="multipart/form-data">
        <label for="title">Title: </label>
            <input minlength="3" maxlength="40" type="text" name="title" required id="title" autocomplete="off"/>
    
            <label for="description">Description <p style="color:rgb(208, 208, 208)">[optional]</p>:</label>
            <textarea maxlength="2000" name="description" rows="4" cols="50" id="description" autocomplete="off"></textarea>
    
            <label for="release_date">Release date: </label>
            <input type="date" name="release_date" required id="release_date" autocomplete="off"/>
    
            <label for="embed">Embed <p style="color:rgb(208, 208, 208)">[recommended size: 500x300] [optional]</p>: </label>
            <input class="upload-btn" type="file" name="embed" id="embed" autocomplete="off"/>
    
            <label for="browser_file">Playable <p style="color:rgb(208, 208, 208)">[ZIP, should contain an index.html file] [optional]</p>: </label>
            <input class="upload-btn" type="file" name="browser_file" id="browser_file" autocomplete="off"/>
    
            <label for="download_file">Downloadable <p style="color:rgb(208, 208, 208)">[should be a ZIP file] [optional]</p>: </label>
            <input class="upload-btn" type="file" name="download_file" id="download_file" autocomplete="off"/>

            <p style="color: white;">Note: All settings can be changed later.</p>

            <button type="submit">Upload</button>
        </form>

        <div id="error_container" class="error-container"></div>
    </div>