Walkie - Talkie

Dears, me and my girlfriend have apple watch nike+ series3. We would like to use walkie-talkie app but I cannot send her invitation. I did it and always watch told me, your invitation to user .... was not send. Could you please help me. We have actual ios on all devices, we reboot iphones and watches as well, we have Facetime in use but still we cannot send invitiation to each other. Thank you for your help