Maintain exchange rate &1/&2 for &3

What causes this issue?

The local currency for the posting is &V2&. For a posting assignment in
CO (for example, a cost center, order, or project), the amount posted
must be convertible to foreign currency &V1&. This is not possible as
no corresponding exchange rate has been maintained.

System Response

This posting is not accepted as long as no exchange rate has been
defined between &V1& and &V2&.

How to fix this error?

Maintain the mean exchange rate of &V1& (foreign currency) to &V2&
(local currency) for &V3& using the customizing function in FI.

Error message extract from SAP system. Copyright SAP SE.