if you want to use mercadopago , At first you need to get your client id and client secrets , you have to setup your mercadopago gateway settings. Go to “Payment settings>Payment Gateway Settings ”.
Credentials are unique passwords that identify an integration in your account. They allow to capture payments in online stores and other applications securely. They can be found on the Dashboard or in your Mercado Pago account by accessing Your Business > Settings > Management and Administration > Credentials.
for more info please visit
https://www.mercadopago.com.br/developers/en/guides/resources/credentials