I tried a IPv6 only VPS. It has no IP v4 address.
It is always error connection when I tried to use an IP v6 to connect it.
Here is some new knowledge for me.
Use a pair  around the IPv6 address when using it in the Browser address .
The correct format is: https://[2001:19f0:8001:ae7:5400:1ff:fe93:a764]:10000
To enabled IPv6 support.
add a line
2) Install Socket6 perl module
apt install libsocket6-perl
Then restart webmin service.
Now I can enter the webmin which is IPv6 only webmin.