Are you including all of the required fields? All I see is that you included the api_key.
Here are the other required fields to make a shipment:
to_address Address required
from_address Address required
parcel Parcel required
Example in PHP:
$to_address = \EasyPost\Address::create(...);
$from_address = \EasyPost\Address::create(...);
$parcel = \EasyPost\Parcel::create(...);
$customs_info = \EasyPost\CustomsInfo::create(...);
\EasyPost\Shipment::create(array(
"to_address" => $to_address,
"from_address" => $from_address,
"parcel" => $parcel,
"customs_info" => $customs_info
));
Source: