DF flag means that an intermediate host (router) cannot fragment the packet if necessary, and it would then need to drop the packet and can send an ICMP message stating that.
RFC 791, Internet Protocol says:
If the Don't Fragment flag (DF) bit is set, then internet
fragmentation of this datagram is NOT permitted, although it may be
discarded. This can be used to prohibit fragmentation in cases where
the receiving host does not have sufficient resources to reassemble
IPv6 does not have the concept of intermediate host fragmentation, so the flag does not exist in the IPv6 header.