AWS codebuild failed at DOWNLOAD_SOURCE phase

I am attempting to create a CICD pipeline using AWS CodePipeline to deploy magento to an EC2 instance and terraform to provision the infrastructure on AWS. After running the script, it fails at DOWNLOAD_SOURCE phase during the build stage. After much enquiry I realised it has to do with VPC settings. In the AWS documentation it says to use private subnet ids and present the arn instead of the just the subnet id. I have done all that but it is still failing. What do I do to resolve this?