Country of declaration &1 not available

What causes this issue?

You tried to create an entry for a declaration country that has not yet
been defined.

System Response

The system issues an error message.

How to fix this error?

Cancel this function.
Create the required declaration country for the corresponding
declaration type on the overview screen. To do this, use the
corresponding function in the navigation bar on the left side of the
screen.
Repeat the function afterwards.

Error message extract from SAP system. Copyright SAP SE.