AWS Dynamodb boto3 batch_get_item ProjectionExpression not working

With the Boto3 Python library, for the below code block while using ProjectionExpression, I am getting an error Requested resource not found.

When I do not use ProjectionExpression, it works but brings all columns.

sellerDict = dynamodb.batch_get_item(
    RequestItems={'Seller':
                   {'Keys': vq},
                   'ProjectionExpression': {
                   'Keys': [{'MobileNo': 'N'}, 
                            {'Offer': 'N'}]
                    }
                  }
)