Hello Vlad
When pinging using IPv6, the router will always use a global unicast address as the source address. This is because link local addresses are not routable. That is, link local addresses cannot be routed beyond the network segment in which they are assigned. In your case, if none of the physical interfaces have global unicast addresses, then it will use the global unicast address assigned to the loopback interface.
Keep in mind that if a router doesn’t have an interface with a global unicast address, the ping will fail. Indeed, the error that appears states that either the address entered is incorrect, or that the appropriate protocol is not running.
According to the ICMPv6 RFC 4443, the choice of the source IPv6 address depends on various factors.
I hope this has been helpful!
Laz