CodeBuild Error: Cannot have more than 0 builds in queue for the account

There are various account level quotas that AWS sets for new and existing accounts. Some of the quotas protect you from unpredicted cost skyrocketing when you just start the cloud journey, the other ones might be applied if AWS detects something unusual in your accounts (e.g. an open vulnerability, an expired credit card, ...)

The best way to resolve this issue is contacting AWS Support and requesting the quota increase for the number of concurrent builds in AWS CodeBuild (you can start e.g. with 5). If there are some things than need to be fixed in your account first, you will be informed about that. Otherwise, the quota will be increased, so you can run your builds in AWS CodeBuild.