Hi
Image names change often so that information is not something that would be documented on the Packer docs. Have your tried using the AWS CLI to list available 2019 Windows Server images?
aws ec2 describe-images --filters "Name=virtualization-type,Values=hvm" --filters "Name=name,Values=*Windows_Server-2019*English*" | jq '.Images[].Name'
Note I am using the jq command to only show the names of the available AMIs. There appears to be a Windows_Server-2019-English-Full-Basee, but you should confirm that it is the AMI you are looking for and that it is available within your selected region using the AWS CLI command.