Omitting the UDP/IPv4 checksum is permitted because the IPv4 header has its own redundant checksum, but omitting the UDP/IPv6 checksum would leave the IPv6 header with no checksum at all, which is why that's not permitted.
https://en.wikipedia.org/wiki/UDP-Lite lets you checksum the UDP/IP header plus an arbitrary (possibly zero-length) prefix of the payload, but I'm not sure if people actually use that.
Omitting the UDP/IPv4 checksum is permitted because the IPv4 header has its own redundant checksum, but omitting the UDP/IPv6 checksum would leave the IPv6 header with no checksum at all, which is why that's not permitted.
https://en.wikipedia.org/wiki/UDP-Lite lets you checksum the UDP/IP header plus an arbitrary (possibly zero-length) prefix of the payload, but I'm not sure if people actually use that.