Scope
I am getting an error [Xero] 1) Account must be valid.2) Tax rate must be valid.

Sync is throwing error - [Xero] 1) Account must be valid.2) Tax rate must be valid

Summary

In Manage Mapping section:

1. Payments & Refunds should be mapped to a bank account from a third party.
2. Invoice line items should be mapped to a revenue/sales account from a third party.

So If any of the above condition is not fulfilled, we will get the below error: 

Solution:

Make sure to use a revenue / sales account under account code