I can't get an IDN domain


Welcome to the forum. InfinityFree owns the domain “rf.gd” so you can’t register it. You can purchase your own domain name somewhere else and then point it to your InfinityFree site:

Or, you can try to create a free InfinityFree account using the subdomain xn–5g8h and select “rf.gd” as your choice of free domain. But, you don’t really “own” this type of domain because InfinityFree owns the Top Level Domain (rf.gd). So, you’re just using their free subdomain.