This article covers the instructions to resolve the sync error 

Error message: Contact - There's a problem with this state, even though it may appear correct. Please select a state from the list of valid states.: Mailing State/Province: MailingState --

Summary:

Sync error may occur while customer entities are synced to Salesforce. This is due to a mismatch in the format of countries listed in Chargebee and Salesforce. Chargebee uses ISO 3166 alpha 2 formats 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:

Update the state names such as Hong Kong, Puerto Rico, American Samoa, U.S. Virgin Islands, Guam, United States Minor Outlying Islands as country names in the Salesforce country picklist. As these are specified as states in the version that Salesforce uses. 

Here is a help document from Salesforce 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. This cannot be changed within Chargebee.