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:~$