External number allocation terminated - External number required

What causes this issue?

External asset numbers are allocated in this class. A pop-up? window is
used for this. You have terminated processing.

System Response

The system cannot create a master record without an asset number. The
system will display the initial request screen again.

How to fix this error?

A. If you want to create an asset in this class, enter an asset number,
or
B. use an asset class which does not have external number allocation

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.