Sim card

Im in croatia and i have an old iphone i bought in united states. i was using another device here in croatia and i want to put the sim card to the iphone. but when i put the sim card inside the device tell me this: the sim card you currently have installed in this iphone is from a service provider that is not supported under the activation policy that is currently assigned by the activation server. this is not a hardware issuenwith the iphone. please insert another sim cardfrom a supported service provider or requiest that this iphone be unlucked by your service provider.