Lambda SAM deploy giving error 'EC2DescribePolicy' at 'policyArn' failed to satisfy constraint: Member must have length greater than or equal to 20

I’m using AWS SAM to deploy a Lambda using cloudformation. The lambda uses the StartInstancesCommand,StopInstancesCommand and DescribeInstancesCommand api to turn instances on and off on schedule.

Using both the EC2DescribePolicy or EC2FullAccessPolicy SAM policy templates, I have been encountering the following error on:

sam deploy

1 validation error detected: Value
‘EC2DescribePolicy’ at ‘policyArn’
failed to satisfy constraint:
Member must have length greater
than or equal to 20 (Service:
AmazonIdentityManagement;
Status Code: 400; Error Code: ValidationError;

How can I fix this error? Do I have to specify ‘policyArn’ manually?

My template.yaml:

AWSTemplateFormatVersion: "2010-09-09"
Transform: AWS::Serverless-2016-10-31
Description: >
  test-function

Resources:
  TestFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: test-function/
      Handler: app.lambdaHandler
      Runtime: nodejs16.x
      MemorySize: 512
      Timeout: 60
      Policies:
        - AWSLambdaBasicExecutionRole
        - AWSLambdaVPCAccessExecutionRole
        - EC2DescribePolicy
      Architectures:
        - x86_64
      Events:
        ScheduledEvent:
          Type: Schedule
          Properties:
            Schedule: cron(0 8 * * ? *)
            Enabled: True

Version information:

$ sam --version
SAM CLI, version 1.60.0
$ node --version
v16.17.0
$ docker --version
Docker version 20.10.17, build 100c701
$ python3 --version
Python 3.9.6

>Solution :

The documentation states:

For every policy template you specify in your AWS SAM template file, you must always specify an object containing the policy template’s placeholder values. If a policy template does not require any placeholder values, you must specify an empty object.

So you should have:

Policies:
  - AWSLambdaBasicExecutionRole
  - AWSLambdaVPCAccessExecutionRole
  - EC2DescribePolicy : {}