Panoptesoft recently removed several AMIs based on older Ubuntu releases (Ubuntu 18.04 LTS Bionic Beaver, Ubuntu 19.04 Disco Dingo, and Ubuntu 19.10 Eoan Ermine). These releases are either no longer supported by Canonical or require security updates for package versions beyond what is available in the supported official repositories.
Existing customers using these images can continue to do so, but are encouraged to update to a newer, supported release such as Ubuntu 20.10 VNC Desktop AMI or Ubuntu 20.04 VNC Desktop AMI.