Terraform Version, subnet_ids Not Recognized

[joswr1ght]

I'm unable to get the sadcloud scripts to deploy following an error in the Cloud Trail module.

(aws) sadcloud (master) $ terraform -version
Terraform v0.15.1
on darwin_amd64
(aws) sadcloud (master) $ git log -1
commit b3de799d960c51503558af749ed41f44efd7d359 (HEAD -> master, origin/master, origin/HEAD)
Author: Rami McCarthy <[email protected]>
Date:   Tue Jun 30 16:14:33 2020 -0400

    Update README.md
(aws) sadcloud (master) $ terraform apply

... some time later:

│ Error: Error launching source instance: VPCResourceNotSpecified: The specified instance type can only be used in a VPC. A subnet ID or network interface ID is required to carry out the request.
│ 	status code: 400, request id: 2f47199d-6ed6-42ce-99e0-aa7982a5df6e
│
│   with module.ec2.aws_instance.main[0],
│   on ../modules/aws/ec2/main.tf line 18, in resource "aws_instance" "main":
│   18: resource "aws_instance" "main" {

Here is the context of the main.tf file:

(aws) sadcloud (master) $ cat -n ../modules/aws/ec2/main.tf | sed -n 17,23p
    17
    18	resource "aws_instance" "main" {
    19	  ami           = data.aws_ami.ubuntu.id
    20	  instance_type = var.disallowed_instance_type ? "t2.micro" : "t2.small"
    21	  subnet_id     = var.main_subnet_id
    22	  count         = var.disallowed_instance_type || var.instance_with_user_data_secrets || var.instance_with_public_ip ? 1 : 0
    23

I'm new to Terraform, and I'm not sure how to respond to this error. When I tried to build I get an error about the use of list() as deprecated, so I converted them to tolist() per the new API use. I've attached my diff, though it doesn't seem to be related to anything for subnets or network IDs. Any ideas? Thanks!

[joswr1ght]

tolist.diff.txt