Which command would you use on an interface to set the IPv6 address?

When you configure routers, always use the rule of major/minor. The major protocol is ipv6, and the minor command is address. So the correct command is ipv6 address 2001:0db8:85aa:0000:0000:8a2e:1343:1337/64. The additional rule is to specify the network portion with a /64.