When you view the list of payment gateways at Configuration > System Settings > Payment Gateways, you may see the following message:

This Payment Solution module could not be loaded, most likely because it has been removed. You should deactivate it now.

You have activated a payment gateway module but the system can't find the file containing the corresponding PHP file in the /modules/gateways directory.

To continue using the payment solution, upload the missing PHP files to the /modules/gateways directory on your server. Then, reload the page.

If you don't need the payment solution, deactivate it.

For more information, see Deactivating Gateway Modules.