OK
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]