can I buy iphone 14 plus in USA an activate it in Croatia?

Yes, if there's a carrier you can use that supports them.

Find wireless carriers and worldwide service providers that offer eSIM service - Apple Support

Additionally:

Make sure it will work in your country.--> 5G and LTE. Find the iPhone that’s right for your country or region. - iPhone - Supported 5G and LTE networks - Apple

Warranty restriction! - Legal - iOS Warranty Document Us - Apple - "IMPORTANT RESTRICTION FOR iPHONE, iPAD AND APPLE TV SERVICE. Apple may restrict warranty service for iPhone, iPad and Apple TV to the country where Apple or its Authorized Distributors originally sold the device." <-- The warranty does not outright deny warranty coverage in other countries but there are many limitations to the effect Apple almost certainly will refuse to do repairs under warranty or even repairs at all except in the country of purchase. The device would have to be sent to a friend in the country of purchase to handle any repair claims because Apple will not handle international shipping.

If your carrier in Croatia is one of these:

Croatia

A1

Hrvatski Telekom

But note that it will have no warranty in Croatia, and you will not be able to get Apple repair service for it in or out of warranty.

As Croatia is a member of the EU you would be much better off getting it in any EU country, because the warranty is valid in all EU countries.