This variable https://github.com/philips-labs/terraform-aws-github-runner/blob/develop/modules/runner-binaries-syncer/variables.tf#L17-L20

variable "distribution_bucket_name" {
  description = "Bucket for storing the action runner distribution."
  type        = string
}

Has this limitation:

Error: Error validating S3 bucket name: only lowercase alphanumeric characters and hyphens allowed in "actions-Default-dist-u9g227runuin7nvex3pysvj9"

Would be good to add some Terraform validation on that variable so we can fail at the plan stage rather than the apply stage

OR

Add the same validation on the variable in the root module instead https://github.com/philips-labs/terraform-aws-github-runner/blob/develop/variables.tf#L22-L25

variable "environment" {
  description = "A name that identifies the environment, used as prefix and for tagging."
  type        = string
}