Diagnosis
On the header overview (historical headers for an alternative), youhave selected a number of headers in order to change or display variousheader data from the headers concerned.
You are now looking at the header detail screen of one of the selectedheaders and want to get data from the previous selected header usingfuncton Header -.
As the header you are looking at is the first selected headerwithin the header overview, this message appears.

Procedure
Choose function Header + to move in the other direction, or goback to the header overview using function key F3 and reselect asrequired.