Cloudformation - adding string in middle

I am trying to concatenate sting in AWS Cloudformation, I am getting error, tried various things . No luck. Enviroment is a Parameter. I am trying to make EndpointIdentifier with join but it doesn't work. Please let me know the correct format. Error message - Template error: every Fn::Join object requires two parameters, (1) a string delimiter and (2) a list of strings to be joined or a function that returns a list of strings (such as Fn::GetAZs) to be joined.

End result should be for e.g if dev is choosen from dropdown - fp-epk-dev-dms-source or fp-epk-dev-dms-target Removed some code which is not required for this question

AWSTemplateFormatVersion: 2010-09-09
Parameters:
 Enviroment:
      Type: String
      Default: "dev"
      AllowedValues:
        - "dev"
        - "uat"
        - "prod"
Resources:
  DMSSourceEndpoint:
    Type: "AWS::DMS::Endpoint"
    Properties:
      EndpointIdentifier: !Join
        - ''
        - 'fp-epk-'
        - !Ref Enviroment
        - '-dms-source'
      EngineName: "ORACLE"
      EndpointType: "source"
      Username: !Ref Username
      Password: !Ref Password
      ServerName: !Ref SourceServer
      Port: !Ref SourcePort
      DatabaseName: !Ref SourceDatabase      
      ExtraConnectionAttributes: "addSupplementalLogging=Y"

  DMSTargetEndpoint:
    Type: "AWS::DMS::Endpoint"
    Properties:
      EndpointIdentifier: !Join
        - ''
        - '-epk-'
        - !Ref Enviroment
        - '-dms-target'
      EngineName: "S3"
      EndpointType: "target"