The error message comes from the Exact Online accounting package.
Please control the following settings in Exact Online:
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
Financial => Analysis & Results => Currency => Exchange rates
Click on the currency code
Can you see the invoice date under the section Exchange rates?
If the invoice date is not listed here, can you Add the new exchange rate?
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
