Obviously, with so many steps being automated for you, there are many things that can potentially go wrong. Packer gives you a few different ways to debug issues with your builds.

One of the most useful arguments to use with Packer is the -debug flag. This will force you to manually confirm each step before it takes place. Doing this makes it easy to work out exactly which step in the command is failing, which, in turn, usually makes it obvious what needs to be changed.

Another useful thing to do is to raise the level of logging output during a Packer command. You can do this by setting the PACKER_LOG variable to true. The easiest way to do this is by using PACKER_LOG=1 at the beginning of your Packer command line. This will mean you get a lot more information printed to the console (for example, SSH logs, AWS API calls, and so on) during the command. You may even want to run with this level of logging normally in your builds for auditing purposes.