You need to show the countries you do and do not wish to post to in your Site Preferences. if these are not set then some may buy but will not be able to pay unless a revised invoice is sent with the correct international postage.
Currently there can be issues revising / amending invoices so best to set your preferences where you wish to post to here:
https://www.ebay.co.uk/ship/prf
On the left sidebar select Postage Preferences, scroll down to Exclude Postage locations and select the countries there you do and do not wish to post to. There is a little box at the bottom of that list to add this to any item listed , with no bids.
Be aware, if you are a new seller, or one returning after a long break, you may not be able to sell internationally straight away.
@pest-5286