Lambda function cannot access S3 bucket, even after granting IAM role/policy

I have followed the instructions for creating an AWS Lambda function and have created an IAM user role for an AWS Lambda function to access an S3 bucket. I have allowed both the putObject and getObject actions for the role, and specified which bucket I wanted Lambda to access within the creation page for the policy. Additionally, I unchecked the "Block all public access" button while creating my S3 bucket. However, whenever I try using boto3:

s3 = boto3.client('s3')
bucket_name = 'example_bucketname'
object_key = 'data.json' 

 try:
        response = s3.get_object(Bucket=bucket_name, Key=object_key)
        cached_data = response['Body'].read().decode('utf-8')
        return json.loads(cached_data)
    except s3.exceptions.NoSuchKey:
        return None

I receive an error stating "An error occurred (AccessDenied) when calling the GetObject operation: Access Denied" when testing my lambda function. Am I forgetting anything?

Any help will be much appreciated. Thank you! (Additionally, I have already looked at the guide posted by AWS OFFICIAL and I have also watched the YouTube video created by Francisco on the AWS OFFICIAL YouTube channel).