Unable to assign a Lambda Layers to a Lambda function

Serverless Framework
1

I am trying at create a lambda layer and assigning it to a lambda function, both the layer and function are defined in same serverless.yml file as follows:

layers:

  wkhtmltopdfbinary:

    path: wkhtmltopdf-binary

    description: "wkhtnltopdf binary"

    compatibleRuntimes: # optional, a list of runtimes this layer is compatible with

      - python3.6

      - python3.7

      - python3.8

    retain: true

functions:

  GenerateInspectionReport:

    handler: handler.GenerateInspectionReport

    name: GenerateInspectionReports

    runtime: python3.6

    memorySize: 128 # optional, in MB, default is 1024

    timeout: 600

    # layers: ${self:custom.layer.${self:custom.stage}}

    layers: 

      - { Ref: WkhtmltopdfbinaryLambdaLayer }

I am getting an error, please help:

Error: The CloudFormation template is invalid: Template format error: Unresolved resource dependencies [WkhtmltopdfbinaryLambdaLayer] in the Resources block of the template
      at provider.request.catch.error (D:\Users\gautam\AppData\Roaming\npm\node_modules\serverless\lib\plugins\aws\deploy\lib\validateTemplate.js:20:13)
      at tryCatcher (D:\Users\gautam\AppData\Roaming\npm\node_modules\serverless\node_modules\bluebird\js\release\util.js:16:23)
      at Promise._settlePromiseFromHandler (D:\Users\gautam\AppData\Roaming\npm\node_modules\serverless\node_modules\bluebird\js\release\promise.js:547:31)
      at Promise._settlePromise (D:\Users\gautam\AppData\Roaming\npm\node_modules\serverless\node_modules\bluebird\js\release\promise.js:604:18)
      at Promise._settlePromise0 (D:\Users\gautam\AppData\Roaming\npm\node_modules\serverless\node_modules\bluebird\js\release\promise.js:649:10)
      at Promise._settlePromises (D:\Users\gautam\AppData\Roaming\npm\node_modules\serverless\node_modules\bluebird\js\release\promise.js:725:18)
      at _drainQueueStep (D:\Users\gautam\AppData\Roaming\npm\node_modules\serverless\node_modules\bluebird\js\release\async.js:93:12)
      at _drainQueue (D:\Users\gautam\AppData\Roaming\npm\node_modules\serverless\node_modules\bluebird\js\release\async.js:86:9)
      at Async._drainQueues (D:\Users\gautam\AppData\Roaming\npm\node_modules\serverless\node_modules\bluebird\js\release\async.js:102:5)
      at Immediate.Async.drainQueues [as _onImmediate] (D:\Users\gautam\AppData\Roaming\npm\node_modules\serverless\node_modules\bluebird\js\release\async.js:15:14)
      at runCallback (timers.js:705:18)
      at tryOnImmediate (timers.js:676:5)
      at processImmediate (timers.js:658:5)
      at process.topLevelDomainCallback (domain.js:126:23)

     For debugging logs, run again after setting the "SLS_DEBUG=*" environment variable.