Looks like a documentation error.
This should be raised as a ticket on the Tor Project Bug Tracker. There are details on the front page for an anonymous login, if required.
I think that it should be of the format:
VirtualAddrNetworkIPv6 [FC00::]/7
The default value, defined in the manual page is specified, in that format, as: [FE80::]/10
VirtualAddrNetworkIPv6 [Address]/bits
When Tor needs to assign a virtual (unused) address because of a
MAPADDRESS command from the controller or the AutomapHostsOnResolve
feature, Tor picks an unassigned address from this range.
(Defaults: 127.192.0.0/10 and [FE80::]/10 respectively.)
Update: I've created a ticket for this so that it's documented on the official bug tracker: #19743 (VirtualAddrNetworkIPv6 manpage entry correction)