To troubleshoot further and find which resources couldn't be found, I tried this to add verbosity:-

study:aws-vpn-vpc $ export TF_LOG=DEBUG
study:aws-vpn-vpc $ terraform plan

Options are TRACE, DEBUG, INFO, WARN or ERROR, or unset TF_LOG which deletes the variable or either VARIABLE= / VARIABLE=“” which just sets it to NULL. Set the following env variable (TF_LOG can be TRACE, DEBUG, INFO, WARN or ERROR):-

TF_LOG=trace terraform init

$ export TF_LOG=DEBUG; terraform plan
2019/03/19 10:50:20 [INFO] Terraform version: 0.11.13  
2019/03/19 10:50:20 [INFO] Go runtime version: go1.11.5
2019/03/19 10:50:20 [INFO] CLI args: []string{"/usr/local/bin/terraform", "plan"}
2019/03/19 10:50:20 [DEBUG] Attempting to open CLI config file: /home/andrew/.terraformrc
2019/03/19 10:50:20 [DEBUG] File doesn't exist, but doesn't need to. Ignoring.

Error: module "lambda": "key" is not a valid argument


2019/03/19 10:50:20 [INFO] CLI command args: []string{"plan"}
2019/03/19 10:50:20 [TRACE] module source: "../../modules/lambda"
2019/03/19 10:50:20 [DEBUG] plugin: waiting for all plugin processes to complete...
$