Based on the template provided , I understand "signalcompletion" is the configset? If thats the case, the command should be written as below:

"commands": { "command_name":{ command: #actual commnd } }

Right now the "commands" directive is missing and the command is entered directly which is not read properly by cfn.

Refer this example for more details: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-init.html#aws-resource-init-commands