Make sure your module is called sanabel_calculator. If it isn't, change sanabel_calculator_menu() to MODULE_NAME_menu(). It's wise to use your module name to prefix all functions that it defines, as a rudimentary namespacing system. You might want to change iban_generator() to sanabel_calculator_generator() as part of that, to avoid confustion in the future.
Return something meaningful from your function. drupal_set_message() can return NULL, and other than that will only ever return a plain array. This is not acceptable as a page callback result, which needs to be a render array (or, less ideally, a string).
Make sure the module is enabled
Clear Drupal's caches
As an aside, consider defining a proper access callback or at least arguments for your page. It may not matter, but it's a good practice to get into.
If it still doesn't work after that, follow the steps again through again. If it still doesn't work, you've got something wrong with your installation. Try to install the module on a fresh copy of Drupal to confirm that.