No such cookbook -- OpsWorks can't find custom cookbook
Even if you've enabled custom recipes have you configured OPsWorks to go to your GIT repo?
http://docs.aws.amazon.com/opsworks/latest/userguide/workingcookbook-installingcustom-enable.html
If your GIT repo is on Github for instance you'll have to also authorize Amazon to access your repo using the proper SSH Key.
Also ensure the nginx has all the dependencies it needs in order to run. It might be easier to just put the cookbooks in a tar.gz and upload it to S3 to use.