You are probably trying to use a Connector Package.
Most likely from the dropdown in the outgoing configuration.
A connector package expects a slightly different payload as it can do more than just send a payload.
Answer
A connector package expects the following:
{
"path" : [],
"query" : [],
"payload" : []
}
Many times it’s enough to copy this and put your existing payload within the “payload” object.
To read more about using connector packages click here.