To enable the Aurthorize.net recurring payment method in nopCommerce, follow steps below:

• Login with Admin credentials.
• Go to Administration section.
• Go to Configuration > Payment > Payment Methods.
• Click the edit link for Authorize.Net payment methods.
• Click Configuration tab.
• If you are going to use Sandbox (test) account, check the sandbox checkbox. Leave it as unchecked if not.
• Select the transaction mode depending upon the requirement. If you want to just authorize the credit card select the Authorize. If you want to have actual payment instantly then go for Authorize And Capture.
• Enter the Transaction key and Login ID which you get from Authorize.net account.
• Click Save button.
• In the System Info tab check the "Active" checkbox to make the payment method active. You can accept credit cards payments.