Выстрадано с шакального видео:
$model = "Боже заработай пж";
$manuf = $_POST['manuf'];
$formFactor = $_POST['formFactor'];
$socket = $_POST['socket'];
$powerPhases = $_POST['powerPhases'];
$ramSlots = $_POST['ramSlots'];
$ramFreq = $_POST['ramFreq'];
$ramMax = $_POST['ramMax'];
$network = $_POST['network'];
$sound = $_POST['sound'];
$sataCount = $_POST['sataCount'];
$m2Count = $_POST['m2Count'];
$fan = $_POST['fan'];
$usb = $_POST['usb'];
$videoOut = $_POST['videoOut'];
$passiveCooling = $_POST['passiveCooling'];
$price = $_POST['price'];
$connect = new mysqli($server, $username, $password, $dbname)
$query = connect->query("INSERT INTO Motherboards (Model, Manufacturer, FormFactor, Socket, Power_phases, RAM_slots, RAM_freq, RAM_max, Network, Sound, SATA, M.2, FAN, USB, Video_out, Passive_cooling, Price) VALUES ('$model','$manuf','$formFactor','$socket','$powerPhases','$ramSlots','$ramFreq','$ramMax','$network','$sound','$sataCount','$m2Count','$fan','$usb','$videoOut','$passiveCooling','$price')");
Проверь не затисалась ли кириллица в имени переменных и перепроверь name="" своих input
