Yes, you can purchase an eSIM for a country you’re not currently in. However, you should only expect it to work when you are physically in that country.
A local eSIM is designed to work only within the selected country. For example, if you buy a local United States eSIM, it will not work while you are in Nigeria. The eSIM will only connect to mobile networks once you arrive in the United States.
This means it’s perfectly fine to buy your eSIM ahead of your trip, but activation and usage will only happen when you’re in the supported location.