Can I travel in Croatia using only Euro?

Officially: NO, because you have to be allowed to convert foreign currency first, if you want to be able to accept one. Often you may find people kindly rejecting Euro and they will explain that they would like to accept it, but it would be against the law.

However, as noted above, some individuals might accept it, as they know nobody will prosecute them.

So just for your understanding, you won't be able to buy anything with Euro in any big shops or government businesses, like public transport etc. You may be able to pay in SOME restaurants, thought.

I am currently travelling in Croatia, and have been for about two weeks. My experience has been that everyone accepts kuna, and some establishments also accept euro. The kuna is effectively pegged to the euro, so exchange rates have been consistent.

In the beginning of my travels here, I needed to buy a bus ticket to the airport, and the woman behind the counter did not accept euros. Luckily there was an ATM nearby, but she seemed to (understandably) feel annoyed that I hadn't bothered to use the country's currency.

My experience is that Euros are coveted there, so in general, nearly all stores and restaurants eagerly take them except government/official stores, like museums. Public transport (buses/ferries) will not take Euros. Of course, the exchange rate you will get from a store or restaurant will be much worse than what you would get at a cambio or bank.