Even with correct credentials, you cannot connect unless your IP is registered
The Yurtiçi Kargo web service looks not only at your credentials but also at which IP address the connection comes from. Your static IP has to be added to Yurtiçi's allow list. Skip that step and the connection fails even with a correct username and password, and the error that comes back usually looks like an authentication failure — people re-check the password over and over, while the problem is the address.
The username and password do not come from Yurtiçi's general customer service but from your sales representative at the regional office you are attached to. Requesting the IP registration from the same person is the shortest route.