After activating Twint with a valid Merchand ID I've got the Error 405 with the following message:
Sorry, we are not supporting the HTTP request method 'GET' on this URL.
Could you please fix that?
I assume you are trying to activate TWINT in the space 14139. Is that correct?
The account is still restricted / not activated and therefore you can not use TWINT in LIVE mode. If the account is active you will be able to enter the Merchant ID without any error message.
I hope this helps.