I want to purchase additional iCloud storage but my country is not listed.

iCloud+ storage subscriptions are simply not available in a number of countries. If you’re in one of them, then it is not available to you.

iCloud+ plans and pricing - Apple Support

Availability of Apple Media Services - Apple Support

Where are you located? If your country is not listed and you can’t use a credit card, it may be that Apple does not do business there. If needed, use the get support link at the top of the page to contact Apple directly for help.

Yes, we understand. If the iCloud+ subscription storage service is not available for your country then you cannot make use of it. It is NOT available in many countries. If you are in a country where it is not available, then you cannot subscribe for more iCloud storage.

Not all Apple services are available in every country, as listed in the links I provided you.

Then you won't be able to purchase it.