It seems that either the checksum file isn't getting uploaded as part of the build process, has the wrong name or has no public access, as can be seen during a routine 'add system image' here.
vyos@vyos:~$ add system image https://s3.amazonaws.com/s3-us.vyos.io/rolling/current/vyos-1.4-rolling-202112300317-amd64.iso Trying to fetch ISO file from https://s3.amazonaws.com/s3-us.vyos.io/rolling/current/vyos-1.4-rolling-202112300317-amd64.iso % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 100 437M 100 437M 0 0 9762k 0 0:00:45 0:00:45 --:--:-- 10.4M ISO download succeeded. Checking SHA256 (256-bit) checksum... % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 --:--:-- 0:00:01 --:--:-- 0 curl: (22) The requested URL returned error: 403 Forbidden Unable to fetch SHA256 checksum file. Do you want to continue without checksum verification? (yes/no) [yes] Checking for digital signature file... % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 --:--:-- 0:00:01 --:--:-- 0 curl: (22) The requested URL returned error: 403 Forbidden Unable to fetch digital signature file. Do you want to continue without signature check? (yes/no) [yes] Checking MD5 checksums of files on the ISO image...OK. Done! What would you like to name this image? [1.4-rolling-202112300317]: OK. This image will be named: 1.4-rolling-202112300317 Installing "1.4-rolling-202112300317" image. Copying new release files... Would you like to save the current configuration directory and config file? (Yes/No) [Yes]: Copying current configuration... Would you like to save the SSH host keys from your current configuration? (Yes/No) [Yes]: Copying SSH keys... Running post-install script... Setting up grub configuration... Done. vyos@vyos:~$