Google "HP 8590 Series Analyzers Service Guide.pdf" and "8590 CLIP 5963-2591.pdf"
From the service manual:
LO UNLVL (U)(H)
Indicates that the analyzer local oscillator distribution amplifier is not
functioning properly. This message may appear on the 8592L, 8593E,
8594E, 8595E, and 8596E with Options 009/010, Option 009, or
Option 010.