How do I make Bitcoin Core listen on IPV6?
Normally it should autodetect. Check for lines like these in debug.log:

AddLocal([2a02:91a:f3:21aa::1]:8333,1)


If that's the case, Bitcoin Core is listening to it, and if another service can't reach it, it may be a network configuration problem or a problem with that service.

If it isn't, you can manually bind to an IP address (IPv4 or IPv6) using the -bind option.