The Vendor you are using has a Currency Code setup on the Vendor Card and this sync error usually happens when the exchange rate is not set up.
You would need to add an Exchange Rate for the currency being used in the effective vendor card.
Please check the vendor and also add a Currency Exchange Rate.
Attached are screenshots for reference:
Once the Currency Exchange Rate is saved, follow these steps:
Go Back to ProcureDesk.
Open the document > Actions.
Click on Send for Integration.