You declared $image_name but used $image1_name
It also looks like you forgot your semicolon on line 7.
if (isset($_POST['add-post'])) {
adminOnly();
$errors = validatePost($_POST);
($_FILES['image']['name']);
$image_name = time() . '_' . $_FILES['image']['name'];
$destination = ROOT_PATH . "/assets/images/" . $image_name;
$result = move_uploaded_file($_FILES['image1']['tmp_name'], $destination);
if ($result) {
$_POST['image1'] = $image1_name;
} else {
array_push($errors, "Failed to upload image");
}
} else {
array_push($errors, "Post image d");
}