Magento 2 Marketplace iyzico Payment Gateway is purely meant for marketplace vendors. If the admin wants to sell his own products on the web store, then he has to register himself as a sub-merchant and get an Admin Sub-merchant id from iyzico Payment Gateway.
For this, the admin has to navigate through Stores->Configuration->Sales->Payment Methods->Iyzico Payment.
The admin has to enter the name, email id, address, international bank account number, merchant name. After entering these details, the admin has to accept iyzico platform user agreement and save.