How to fix Error: Unsuitable value type - list of string required in Terraform

First, identify which argument or variable is causing the error. Check the error message and the resource documentation to understand what type it expects.

If the error says 'list of string required', that means the argument should be a list of strings like:

["value1", "value2", "value3"]

Check the provider documentation or resource schema for the specific argument that's failing.

Look at your configuration and check if you're wrapping a list in extra brackets.

WRONG (extra brackets):
instances = [var.instance_list]

CORRECT (use the list directly):
instances = var.instance_list

If you want to combine or manipulate lists, use list functions or the concat() function instead of wrapping.

In your variables.tf, ensure that any variable you're passing to a list-of-strings argument is actually defined as list(string).

Example of a properly defined list variable:

variable "allowed_users" {
type = list(string)
description = "List of allowed usernames"
}

If you have a single string that needs to become a list, either:
1. Define the variable as list(string) and wrap the value in square brackets when using it
2. Wrap the single value in a list at the point of use: allowed_users = [var.single_user]

If you have a single string value that needs to be used where a list of strings is expected, convert it to a list:

variable "single_user" {
type = string
}

resource "example_resource" "main" {
# Convert single string to a list of one string
users = [var.single_user]
}

Or use Terraform functions to manipulate the data:

resource "example_resource" "main" {
# Use tolist() to ensure it's a list
users = tolist([var.single_user])
}

If you need to apply a list of values to multiple resources or arguments, use for_each or count with proper list indexing:

variable "users" {
type = list(string)
}

resource "example_resource" "by_user" {
for_each = toset(var.users)

# Each iteration gets one string from the list
username = each.value
}

Run terraform validate to check for syntax errors before applying:

terraform validate

If you want to see what types Terraform has determined for your values, use terraform console:

terraform console
> var.my_variable
> type(var.my_variable)

This helps diagnose type mismatches quickly.