Not sure what is wrong with the json
This is the workflow
same here.. problem with the http request
Escape as well the double quotes " inside the expression as you can see you have double quotes, doubled.
But you already use JSON.stringify() that does that.
Remove stringify , if you wanna handle the escapes, or remove replace.
where is your access token?
This is the body json script. but mine is have same error
{
“recipient”: {
“id”: “{{ $(‘Webhook’).item.json.body.entry[0].messaging[0].sender.id }}”
},
“messaging_type”: “RESPONSE”,
“message”: {
“text”: “{{ $json.output }}”
}
}
{{ JSON.stringify($json.output) }} should be in your case @Mavz_Ocampo .
And as summary @NUNEZ_JOSHUA_S either use JSON.stringify() or .replace()
Do you see “” sign at your output :
“text”: ""Hello 
How…
You have doubled the " quotes sign. That extra " sign breaks your json in your request.
if you can understand filipino you can check Koya RJ in youtube.
},
“messaging_type”: “RESPONSE”,
“message”: {
“text”: {{ JSON.stringify($json.output.replace(/\n/g, ‘\n’)) }}
}
this is mine. is use this for printing services inquiry. knowlegebase is the price and some instruction for ai agent, sheet is for record for customer details and orders.
None of you understood what i said.
Soory, cannot help you further, since clearly 
is hard to get that " that sign breaks json.