I would try to check if there are other Android browsers where the submit button works.

I guess this works the same way, either way, please try

<input type="submit" value="Submit">

I’ve found a solution to the problem. After deleting one particular CSS style on my button, the form started working!

 .contact-box > .contact-box-nazev > form > button:focus
 {
   display: none;
   z-index: 99
 }

I’ve encountered a problem that’s quite annoying. I tried to look up some information about the topic and tried some code that was supposed to fix it but, unfortunately, no luck. Some people say it should be CSS problem, others say it’s form problem. None of those answers seem to work with me. I don’t know why. I am sending you my CSS and HTML form for you guys to check it out for me. Hopefully you will find some problem and explain it to me.

HTML:

 <div class="contact-box">
            <div class="contact-box-nazev">
                <h1>ADD VIDEO</h1>
                <p>ADD A VIDEO TO SEE IT ON THE MAIN PAGE</p>
                <form action="addvideo.php" method="POST" enctype="multipart/form-data">
                    <input type="text" id="logoname" name="logoname" placeholder="LOGO NAME"><br>
                    <input type="file" id="myfile" name="myfile"><br>
                    <input type="text" id="thumbname" name="thumbname" placeholder="THUMBNAIL NAME"><br>
                    <input type="file" id="mythumbnail" name="mythumbnail"><br>
                    <input type="text" id="url" name="url" placeholder="VIDEO URL"><br>
                    <input type="text" id="bck" name="bck" placeholder="BACKGROUND COLOR (HEX)"><br>
                    <input type="text" id="txt" name="txt" placeholder="TEXT COLOR (HEX)"><br>
                    <input type="text" id="comment" name="comment" placeholder="COMMENT"><br>
                    <button type="submit" name="submit">ADD A VIDEO</button>
                    <p>or delete any existing video <a href="deletevideo.php">here</a></p>
                    <?php 
                        error_reporting(0);
                        foreach($errors as $error):
                            echo "<p style='color: red;'>".$error."</p>";
                        endforeach;
                    ?>
                </form>
            </div>
    </div>    

CSS:

    .contact-box
{

    width: 50%;
    margin: 0 auto;
    height: auto;
    padding: 10px;
    margin-bottom: 150px;
    z-index: 5;
}

.contact-box > .contact-box-nazev
{
    color: black;
}

.contact-box > .contact-box-nazev > h1
{
    text-align: center;
    font-family: blackex;
    font-size: 75px;
    width: 100%;
    margin: 0;
}

.contact-box > .contact-box-nazev > p
{
    color: BLACK;
    text-align: center;
    font-size: 13px;
    font-family: regular;
}

.contact-box > .contact-box-nazev > form
{
    text-align: center;
    width: 60%;
    margin: 30px 0 30px 0;
    margin: 0 auto;
}

.contact-box > .contact-box-nazev > form > input[type=text]
{
    padding: 5px 20px 5px 20px;
    border-radius: 0px;
    margin: 10px;
    width: 55%;
    background-color: #d2d2d2;
    border: none;
    color: #7a7a7a;
    font-family: regular;
    transition: .5s;
}

.contact-box > .contact-box-nazev > form > input[type=password]
{
    padding: 5px 20px 5px 20px;
    border-radius: 0px;
    margin: 10px;
    width: 55%;
    background-color: #d2d2d2;
    border: none;
    color: #7a7a7a;
    font-family: regular;
    transition: .5s;
}

.contact-box > .contact-box-nazev > form > input[type=password]:focus
{
    background-color: black;
    color: #e9e9e9;
    outline: none;
}

.contact-box > .contact-box-nazev > form > input[type=text]:focus
{
    background-color: black;
    color: #e9e9e9;
}

.contact-box > .contact-box-nazev > form > input[type=text]:focus
{
    outline: none;
}

.contact-box > .contact-box-nazev > form > textarea
{
    position: relative;
    resize: none;
    margin: 10px;
    width: 55%;
    background-color: #d2d2d2;
    border: none;
    color: #7a7a7a;
    padding: 5px 20px 5px 20px;
    font-family: regular;
    transition: .5s;
    border: 0px;
    border-radius: 0;
}

.contact-box > .contact-box-nazev > form > textarea:focus
{
    background-color: black;
    color: #e9e9e9;
}

.contact-box > .contact-box-nazev > form > button
{
    padding: 5px 20px 5px 20px;
    margin: 10px;
    width: 55%;
    border: none;
    background-color: #ff0026;
    color: white;
    font-family: regular;
    transition: .5s;
    z-index: 99;
}

.contact-box > .contact-box-nazev > form > button:hover
{
    background-color: black;
    color: white;
    z-index: 99
}

.contact-box > .contact-box-nazev > form > button:focus
{
    display: none;
    z-index: 99
}