User is not authorized to perform: dynamodb:PutItem on resource

The dynamoDB access denied is generally a Policy issue. Check the IAM/Role policies that you are using. A quick check is to add

AmazonDynamoDBFullAccess 

policy in your role by going to "Permissions" tab in AWS console. If it works after that then it means you need to create a right access policy and attach it to your role.

Check the access key you are using to connect to DynamoDB in your Node app on AWS. This access key will belong to a user that does not have the necessary privileges in IAM. So, find the IAM user, create or update an appropriate policy and you should be good.

For Beanstalk you need to setup user policies when you publish. Check out the official docs here.

And check out the example from here too, courtesy of @Tirath Shah.

In my case (I try to write to a DynamoDB table through a SageMaker Notebook for experimental purposes), the complete error looks like this:

ClientError: An error occurred (AccessDeniedException) when calling the UpdateItem operation: User: arn:aws:sts::728047644461:assumed-role/SageMakerExecutionRole/SageMaker is not authorized to perform: dynamodb:UpdateItem on resource: arn:aws:dynamodb:eu-west-1:728047644461:table/mytable

I needed to go to AWS Console -> IAM -> Roles -> SageMakerExecutionRole, and Attach these two Policies:

AmazonDynamoDBFullAccessAWSLambdaInvocation-DynamoDB

In a real-world scenario though, I'd advise to follow the least-permissions philosophy, and apply a policy that allows put item method to go through, in order to avoid accidents (e.g. deleting a record from your table).