I'm creating a custom payment method using example plugin, but during checkout, I get an error 30 - Failed Missing Field Merchant ID.
Has anyone encountered this? Can someone help? Where should this field be?
The custom extension that you are using requires you to have a merchant ID (like any payment processor), but it seems that you have not provided it with one. This is typically done at the settings/options level of the extension. If you can't find it, then you should contact the developer. If you did set it and it's still returning the same error, then most likely there is a bug in the code somewhere.
It seems that you are using some credit card payment plugin.
The best thing to do, is to read the documentation.
But anyway, you'll need the bank info about this (go to your bank services provider and ask for them).