You get this error when the country field is disabled/removed in either billing or shipping section in the plugin settings. When you provide a validation for state field or zip code field in any section, it is mandatory to have an active country field. But, there is an option for you to hide the country field if you don't feel like displaying it.
For this, you can create a new field in that particular section in the same name as billing_country/shipping_country. This will override any field which is existing in the same name. You can set the field type as hidden if you want to hide the field in the checkout page. Then you have to provide a country code as a Default Value so that the section will always consider the given country as default. For example, if you want to keep the United States of America as your default billing/shipping country, you can give the code as 'US'.