Am I missing required fields for this INSERT?

Question

Something is wrong with my CardID in my INSERT Statement. Am I overlooking something that needs to be included? I am not getting an error, it just says there's an error on function bind_param()

I copy pasted the code from another page and my thought is that there are too many characters in the random cardid. The error did not appear until I tried to add the cardid to the insert.

Here's the table and the code.

// create the conditions for a random cardID
        $CardIDLength = 20;
        $CardIDString = "123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
        $CardIDDuplicate = true;
        // make a random value for $cardID and then see if it exists in the DB
        while ($CardIDDuplicate = true) {
            $CardID = substr(str_shuffle($CardIDString), 0, $CardIDLength); //shuffle String, start with 0, 11 characters long
            $CardIDSQL = "SELECT CardID FROM FlashcardTable WHERE CardID = ?";
            $CardIDSTMT = $conn->prepare ($CardIDSQL);
            $CardIDSTMT->bind_param("s", $CardID);
            $CardIDSTMT->execute();
            $CardIDRESULT = $CardIDSTMT->get_result();
            $CardIDROW = $CardIDRESULT->fetch_assoc();                
            // Check table with search to find out if there are any values that match the one generated above
            $CardIDRESULTCheck = mysqli_num_rows($CardIDRESULT);
            if ($CardIDRESULTCheck <= 0) {
                $CardIDDuplicate = false;
                break;
            }
            else {
                // repeat until you have a unique ID
            }               
        }
        $CardIDSTMT->free_result();
        $CardIDSTMT->close();

$CardOrder = -1; // -1 = setname only, no cards created when the setname was initialized 
        $CardSQL = "INSERT INTO FlashcardTable (PostID, UserID, SetName, CardID, CardOrder, Visibility, Date) VALUES (?, ?, ?, ?, ?, ?)";
        $CardSTMT = $conn->prepare ($CardSQL);
        $CardSTMT->bind_param("ssssiss", $PostID, $UserID, $SetName, $CardID, $CardOrder, $Visibility, $Date);
        $CardSTMT->execute();
        $CardSTMT->free_result();
        $CardSTMT->close();