AWS Target Group unable to add multiple targets

I am trying to create ALB and Web Servers by using modules. While Web Servers and ALB gets created, I am unable to add the web servers as targets to the target group. The first one gets attached while the 2nd one fails.

Error: Error registering targets with target group: ValidationError: Instance ID ' i-0cf0a85c8866214ca' is not valid
        status code: 400, request id: 208778c2-c939-4ee5-a134-1434d1132903

Below are my code snippets:

resource "aws_alb_target_group_attachment" "tg_attach" {
  count =
  target_group_arn = aws_alb_target_group.front_end_tg.arn
  port = 80
  target_id = element(split(",", var.web_server_id), count.index)


output "web_server_id" {
  value = join(", ", aws_instance.web.*.id)


#Deploy Application Load Balancer
module "load_balancer" {
  source         = "./load_balancer"
  alb_depends_on = [module.web_servers]
  pubsubnets     = module.networking.public_subnets
  alb_source     = module.networking.alb_source
  alb_perf       = module.networking.alb_perf
  web_server_id  = module.web_servers.web_server_id
  perf_vpc       = module.networking.vpc_id
  tg             = var.web_count


web_count     = 2

Could anyone please advise how do I make the error go away?

AWS Console