Hurricane IPv6 buffer space error

Problem 1: the /64 on the end of the address parameter is incorrect. That's what the netmask parameter is for; you should delete the /64 on the end of address.

However, the error you're seeing is what you'd expect if there was already a tunnel configured with the name "he-ipv6". At a guess, you've already run ifup he-ipv6, and it's got as far as creating the tunnel before falling over with an error due to the trailing /64. Try doing

sudo ip tun del he-ipv6

to delete the half-configured tunnel before trying again.