In Server1: (Auto Ethernet) is the default connection, which was available on the system, so I am modifying it, instead of adding new connection:

nmcli connection modify “Auto Ethernet” ipv6.method manual ipv6.addresses fddb:fe2a:ab1e::c0a7:64/64

systemctl restart NetworkManager

In desktop1: (Auto Ethernet) is the default connection, which was available on the system, so I am ust modifying it, instead of adding new connection:

nmcli connection modify “Auto Ethernet” ipv6.method manual ipv6.addresses fddb:fe2a:ab1e::c0a8:64/64

systemctl restart NetworkManager

Try ping to itself:
ping6 fddb:fe2a:ab1e::c0a8:64

Try ping to server1:
ping6 fddb:fe2a:ab1e::c0a7:64