Unable to launch multiple EC2 instances in different availability zones using terraform
It looks like you're using the wrong brackets? I think you need to use a list:

variable "azs" {
  description = "Run the EC2 Instances in these Availability Zones"
  type = "list"
  default = ["us-east-1a", "us-east-1b", "us-east-1c"]
}


Source