Cloudformation Transform invalid Yaml/JSON

I have 2 templates as below. What I want to ask is how can I use Sub and other variables in Transform?

When I add UserData, I get the following error. This is about Sub. How can I change it according to the transform?

Transform AWS::Include failed with: The specified S3 object's content should be valid Yaml/JSON. Rollback requested by user.

main.yaml

Parameters:
  VpcId:
    Type: 'AWS::EC2::VPC::Id'
  Subnets:
    Type: 'List<AWS::EC2::Subnet::Id>'
  InstanceType:
    Type: String
    Default: c5.large
    AllowedValues:
      - c5.large
      - c5.xlarge
Resources: 
 Instance:
    Fn::Transform:
      Name: AWS::Include
      Parameters:
        Location:
          Fn::Sub: s3://test-bucket/instance.yaml

instance.yaml

Type: 'AWS::EC2::Instance'
Properties:
  KeyName: test
  ImageId: ami-0015a39e4b7c0966f
  InstanceType:
    Ref: InstanceType
  SubnetId:
    Fn::Select: [ 0, Ref: Subnets ] 
  SecurityGroupIds: 
    - Fn::GetAtt: [ Sec, GroupId ]
  UserData:
    Fn::Base64:
      !Sub |
        #!/bin/bash -xe
        yum update -y aws-cfn-bootstrap
        /opt/aws/bin/cfn-init -v --stack ${AWS::StackName} --resource LaunchConfig --configsets wordpress_install --region ${AWS::Region}
        /opt/aws/bin/cfn-signal -e $? --stack ${AWS::StackName} --resource WebServerGroup --region ${AWS::Region}

You can't use !Sub. It should be Fn::Sub::

Type: 'AWS::EC2::Instance'
Properties:
  KeyName: test
  ImageId: ami-0015a39e4b7c0966f
  InstanceType:
    Ref: InstanceType
  SubnetId:
    Fn::Select: [ 0, Ref: Subnets ] 
  SecurityGroupIds: 
    - Fn::GetAtt: [ Sec, GroupId ]
  UserData:
    Fn::Base64:
      Fn::Sub: |
        #!/bin/bash -xe
        yum update -y aws-cfn-bootstrap
        /opt/aws/bin/cfn-init -v --stack ${AWS::StackName} --resource LaunchConfig --configsets wordpress_install --region ${AWS::Region}
        /opt/aws/bin/cfn-signal -e $? --stack ${AWS::StackName} --resource WebServerGroup --region ${AWS::Region}

This limitation is covered in the documentation:

We don't currently support using shorthand notations for YAML snippets.