Trouble implementing validator for MongoDB Schema

Question

platform: [{
    validate: {
      validator: array => { //our custom validator, array is the provided array to be validated
        const filtered = array.filter((obj, index, self) => self.findIndex(el => el.platformName === obj.platformName) === index); //this removes any duplicates based on object key
        return array.length === filtered.length //returns true if the lengths are the same; if the lengths aren't the same that means there was a duplicate key and validation fails
      },
      message: 'Detected duplicate keys in {VALUE}!',
    },
    platformName: {
      type: String, 
      enum: Platform, //category must be in this enum
    },
    link: {
      type: String, 
    },
  }],

I'm using the above method to get something like

platform: [{"platformName": "FACEBOOK", "link": "DJNgJD"}, {"platformName": "MEETUP", "link": "DJNgJD"}]

and trying to use this validator I got from stack overflow it self to prevent duplicate key values for platformName key, but getting the error below. link to stack overflow answer i got this code from How to validate object keys and values in Mongoose Schema?

throw new Error('`' + firstPieceOfPath + '` may not be used as a schema pathname');

Using Ts btw, someone help me figure out what I'm doing wrong