PHP move_uploaded_file() error?

Edit the code to be as follows:

// Upload file
$moved = move_uploaded_file($_FILES["file"]["tmp_name"], "images/" . "myFile.txt" );

if( $moved ) {
  echo "Successfully uploaded";         
} else {
  echo "Not uploaded because of error #".$_FILES["file"]["error"];
}

It will give you one of the following error code values 1 to 8:

UPLOAD_ERR_INI_SIZE = Value: 1; The uploaded file exceeds the upload_max_filesize directive in php.ini.

UPLOAD_ERR_FORM_SIZE = Value: 2; The uploaded file exceeds the MAX_FILE_SIZE directive that was specified in the HTML form.

UPLOAD_ERR_PARTIAL = Value: 3; The uploaded file was only partially uploaded.

UPLOAD_ERR_NO_FILE = Value: 4; No file was uploaded.

UPLOAD_ERR_NO_TMP_DIR = Value: 6; Missing a temporary folder. Introduced in PHP 5.0.3.

UPLOAD_ERR_CANT_WRITE = Value: 7; Failed to write file to disk. Introduced in PHP 5.1.0.

UPLOAD_ERR_EXTENSION = Value: 8; A PHP extension stopped the file upload. PHP does not provide a way to ascertain which extension caused the file upload to stop; examining the list of loaded extensions with phpinfo() may help.

Check that the web server has permissions to write to the "images/" directory

Try this:

$upload_dir = $_SERVER['DOCUMENT_ROOT'] . "/images/";

if (is_dir($upload_dir) && is_writable($upload_dir)) {
    // do upload logic here
} else {
    echo 'Upload directory is not writable, or does not exist.';
}

This will instantly flag any file permission errors.

How can I know that what is the problem

Easy. Refer to the error log of the webserver.

how can I get the actual problem to display to the user ?

NEVER do it.
An average user will unerstand nothing of this error.
A malicious user should get no feedback, especially in a form of very informative error message.

Just show a page with excuses.

If you don't have access to the server's error log, your task become more complicated.
There are several ways to get in touch with error messages.

To display error messages on screen you can add these lines to the code

ini_set('display_errors',1);
error_reporting(E_ALL);

or to make custom error logfile

ini_set('log_errors',1);
ini_set('error_log','/absolute/path/tp/log_file');

and there are some other ways.
but you must understand that without actual error message you can't move. It's hard to be blind in the dark