If one of your journals is locked, the field here would be selected. To unlock a journal, first highlight the locked journal, then press the Enter key or double click the left mouse button. These are the journals that may or may not be locked:
A/P DIST
A/R DIST
BNK DIST
CO DIST
FA DIST
GENERAL
IC DIST
INV DIST
Multi-Site
PC DIST
PO DIST
PR DIST
SF DIST
WC DIST
Also, any user-defined journal may or may not be locked.