I would like to ask how can I add payment in the shopping cart?
Which payment method do you want to use and which Magento version are you using?
The payment method is our own and the version 2.0.11, thank you for your reply
Maybe you have some error on the code (I can't know).
I can recommend you the the serie about payments from Max Pronko (https://www.maxpronko.com/magento-2). You'll find a lot of useul information about this topic on his blog.
Do you have a merchant Virtual terminal?