By default, IAM Users do not have access to the billing section of the AWS console. You must perform these steps using the root login details for your account, or enable IAM access for other users, which is a one-off step.
While you should not generally use the root credentials for your AWS account when administering, creating budgets (which should happen only infrequently) is an exception.
You should not be creating access keys for your root account under any circumstances, which is another reason why we use the console (and not the CLI) for this recipe.