The error message comes from the Exact Online accounting package.
Please control the following 2 settings in Exact Online:
1. Financial => Analysis & Results => Currency => Exchange rates
Select the option for: Use the exchange rates from the current company
Do not use the option: Use the exchange rates from the company
2. Settings => Financial => Journals => Purchase
Is there a check-mark next to "Variable: currency?
If no, select this field by putting a check-mark
Is there also a check-mark for "Variable: Exchange rate?
If yes, de-select this field so that it is not selected