Cloudformation issue with target groups and ASG
Resources:                                                                                                                                                                                  
    TargetGroup:                                                                                                                                                                              
        Type: AWS::ElasticLoadBalancingV2::TargetGroup                                                                                                                                        
        Properties:                                                                                                                                                                           
          VpcId:                                                                                                                                                                              
            Fn::ImportValue:                                                                                                                                                                  
              !Sub "${Prefix}-VpcId"                                                                                                                                                          
          TargetType: instance                                                                                                                                                                
          Port: 443                                                                                                                                                                           
          Protocol: HTTPS                                                                                                                                                                     
    AutoScalingGroup:                                                                                                                                                                         
        Type: AWS::AutoScaling::AutoScalingGroup                                                                                                                                              
        Properties:                                                                                                                                                                           
          AutoScalingGroupName: Autoscaling Group Alpha                                                                                                                                       
          Cooldown: "120"                                                                                                                                                                     
          DesiredCapacity: "1"                                                                                                                                                                
          LaunchConfigurationName: AlphaLaunchConfiguration                                                                                                                                   
          MaxSize: "1"                                                                                                                                                                        
          MinSize: "1"                                                                                                                                                                        
          Tags:                                                                                                                                                                               
            -                                                                                                                                                                                 
              Key: Name                                                                                                                                                                       
              Value: Alpha ASG                                                                                                                                                                
              PropagateAtLaunch: true                                                                                                                                                         
          TargetGroupARNs:                                                                                                                                                                    
            - Ref: TargetGroup


As the error message states, TargetGroupARNs value should be of type List.
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-as-group.html#cfn-as-group-targetgrouparns