Error: Missing resource instance key

  on emr.tf line 125, in resource "aws_cloudwatch_metric_alarm" "EMR-Master-High-RootDiskUtil":
 125:   count = length(data.aws_instances.emr_instances_master.ids)

Because data.aws_instances.emr_instances_master has "count" set, its
attributes must be accessed on specific instances.

For example, to correlate with indices of a referring resource, use:
    data.aws_instances.emr_instances_master[count.index]

resource "aws_cloudwatch_metric_alarm" "EMR-Master-High-RootDiskUtil" {
  for_each = toset(data.aws_instances.emr_instances_master.ids)
  ...
  dimensions = {
    InstanceId = each.value
    ..
  }
}