ℹ
info HP
Status Power Button Lockout
The power button does not function to turn the monitor on or off.
? What Causes This
Lockout feature enabled by holding the power button for ten seconds.
ⓘ What This Means
Hold the power button down for 10 seconds to unlock it.
Source: page 69 of the official manual
From the manual
This error code appears in:
HP Z27 27 Inch 4K Uhd Display