Lambda and DynamoDB : is not authorized to perform: dynamodb:Scan

As already stated, you need to add the permissions to your serverless definition.

The docs are quite extensive on this topic: serverless IAM guide

In you case, you probably just need to add something like the following permission to your serverless.yml.

provider:
  iamRoleStatements:
    -  Effect: "Allow"
       Action:
         - "dynamodb:Scan"
       Resource: "arn:aws:dynamodb:us-east-1:245912153055:table/Pets"