postcode for billing address

Here's an Apple Recommended reply from a similar thread What is Lebanon postal code - Apple Community

"In a browser search/URL box, type 'what is the post code for [insert your location]' and search for the post code. "

Another easy way is to look at an item of mail you have received. Typically you will see a series of numbers, or numbers and letters. This is better than simply searching for a city since in the USA at least some cities are divided into many postcodes.

Don't you have postal/zip codes in Lebanon? If so, you can keep it empty. If this is not working Contact - Official Apple Support for further assistance