Ticket creation automation > Webhook > Update ticket via API

So, I created automation rule on ticket creation. This rule triggers webhook and send ticket details to my ecommerce website endpoint. Ticket content (subject, email, content) are being processed, compared against my ecommerce existing data and based on all this, I’m trying to assign ticket to group/agent, set priority etc. But, when I try to do ticket update via api, I receive a message: 'POST method is not allowed. It should be one of these method(s): GET, PATCH, PUT, DELETE'
Ticket update via api, normaly, works just fine in all other cases. Only in this scenarion it will not work. Why? Is it some kind of loop in here? And what is the alternative?