Error on making plan

Hi,
I am getting below error.

│ on .terraform/modules/tableau_ssm_automation.cluster_master/modules/ssm_step/main.tf line 22, in locals:
│ 22: }[local.file_ext]
│ ├────────────────
│ │ local.file_ext is “yml”
│ │ var.os is “windows”

│ The given key does not identify an element in this collection value.


│ Error: Invalid index

│ on .terraform/modules/tableau_ssm_automation.cluster_master/modules/ssm_step/main.tf line 28, in locals:
│ 28: }[local.file_ext]
│ ├────────────────
│ │ local.file_ext is “yml”
│ │ var.os is “windows”

│ The given key does not identify an element in this collection value.

What could be the reason for it. Can somebody connect live if possible?