You can make use of this guide when you get the below sync error. 

"Contact:There's a problem with this country, even though it may appear correct. Please select a country/territory from the list of valid countries.: Mailing Country:MailingCountry --"

Summary:

Sync errors while customer entities are synced with Salesforce. This error happens due to a mismatch of the format of countries listed in Chargebee and Salesforce. Chargebee uses ISO 3166 alpha 2 format of country naming convention. Salesforce uses a different version of this. Hence this throws an error for countries that are listed as states in the format used by Salesforce.

Solution:

For example, Viet Nam, Palestinian Territory (Occupied) and Côte D'ivoire respectively(hereby names mentioned in the ISO 3166 alpha 2 format) are to updated as a country within Salesforce country picklist. The exact match is looked at while the data sync happens. 

You can make use of the attached list for the country lists used. Here is a help document for more information on updating the picklist.

The country code format that we use is ISO 3166 alpha 2 format and is being globally used across our system.