Since 1994 the official currency of Croatia is the Kuna (divided into 100 Lipa). The name dates back to the time when the fur of martens had been used as a currency – the Croatian word for marten is kuna. So you’re paying with martens in Croatia. 😉 But that will all end in 2023 when the Euro becomes the currency of the country. Fortunately the marten will stay – it is shown on the Croatian Euro coins.
At Dubrovnik getting cash is not a problem at all. First, as in many European countries, you can pay nearly everywhere by card. Second there are ATMs everywhere and withdrawing cash is absolutely hassle-free. If you think you’ve seen the Kuna bills before then you’re probably a middle-aged or older German: the style is very close to the last series of Deutsche Mark.