That seems to be something before the installer is even passed to bash.
Ubuntu 19 is unsupported, we support 16, 18 and 20. Offhand I don't see what would prevent us from supporting 19, and if we can then we'll just need to update the TXT record and you'll be able to install.
In the meantime you can try
curl -sSL https://install.pi-hole.net | PIHOLE_SKIP_OS_CHECK=true sudo bash
without the export to see if that helps.
Edit: 19.10 is EOL so it probably will not be added to our supported list. Ubuntu 19.10 Hits End of Life, Users Should Upgrade Now - OMG! Ubuntu