Webhook is a means to receive the result after sending a message. I understood that the implemented bot succeeded in sending message, but it would be better to separately check whether it is possible to connect to the implemented webhook's endpoint (https://PROJECTNAME.herokuapp.com/callback). The simplest way is to check what happens when you call the endpoint of the webhook directly from your PC.
did you configure the variables in heroku app? you should configure the following from heroku config vars. channel access token channel secret