Please follow the general troubleshooting steps first:

Update your copy of alertmanager-sns-forwarder to the latest version from github. with git fetch https://github.com/DataReply/alertmanager-sns-forwarder master && git reset --hard FETCH_HEAD
Delete the alertmanager-sns-forwarder binary from your PATH, and install locally built binaries.

Bug reports:

Fill in the following sections with explanations of what's gone wrong.

Steps you follow to reproduce the error:

Your steps here.

What did you expect to happen instead:

Your answer here.

What's your theory on why it isn't working:

Your theory here.