Hi, @MEiji Thanks for reaching out. If you have access to support (you included the Professional tag) I’d suggest filing a support ticket.
Questions:
Are you using the endpoint — Associate a ticket with another object PUT /crm/v3/objects/tickets/{ticketId}/associations/{toObjectType}/{toObjectId}/{associationType}
If you make a test request from the API documentation (above) using your Private App, do you get the same error?