Because TeslaSCADA2 it's a cross platform solution it supports OPC UA protocol. TeslaSCADA doesn't support classic OPC because it works only on Windows. According information about OPCtechs on their site, it doesn't have OPC UA solution. You can use OPC UA wrapper to convert classic OPC into OPC UA. Or you can use OPC UA servers for your devices. Also you can check that TeslaSCADA2 supports devices you use in your project.
OPC UA server URI it's an address of OPC UA server.