In most cases, no, you can’t refill Elf Bar 600. They are designed to be single-use, which means that once the e-liquid is finished, you need to throw the device away and open a new one!
The exception to this is the Elfa range by ElfBar, which features replaceable cartridges to allow you to refill your vape in a range of different flavours, without having to buy an entirely new vape device.