dhcpcd-discuss

Hi!
I am trying to get IPv6 prefix delegation with dhcpcd. But when I do so, it fails to assign it to my internal interface:
dhcpcd[1814]: version 6.3.2 starting
dhcpcd[1814]: DUID <duid>
dhcpcd[1814]: ppp0: IAID 00:00:00:00
dhcpcd[1814]: ppp0: IAID 00:00:00:01
dhcpcd[1814]: ppp0: soliciting a DHCPv6 lease
dhcpcd[1814]: ppp0: ADV 2003:xx:xxxx:xxxx::/56 from fe80::xxx:xxxx:xxxx:1439
dhcpcd[1814]: ppp0: REPLY6 received from fe80::xxx:xxxx:xxxx:1439
dhcpcd[1814]: enp5s0: interface does not exist for delegation
dhcpcd[1814]: ppp0: renew in 900 seconds, rebind in 1440 seconds
dhcpcd[1814]: forked to background, child pid 1842

Why? What does this error exactly mean? The interface name is correct and it does exist.
Could somebody explain this to me?

Thanks!
Conrad