You can only communicate with a buyer if he/she is your “transacting partner” i.e. an eBay user that has committed to purchasing your item but is yet to pay for it.
Here’s how you can contact your transacting partner:
In My eBay, select Sold – click on “Contact Buyer” option available on the right-hand side
In Seller Hub, select Orders
Find the items for which you’re wanting to contact the buyer
Click on the drop-down available under the “Actions” tab
Click on “Contact Buyer”