Nikon D780 Reference Manual page 260

D
Time Restrictions on Measuring Preset Manual White Balance
(Viewfinder Photography)
If no operations are performed during viewfinder photography while the
displays are flashing, direct measurement mode will end in the time
selected for Custom Setting c2 [Standby timer].
A
Measuring Preset Manual White Balance (Viewfinder
Photography)
Preset manual white balance cannot be measured during time-lapse
movie recording or while you are shooting a multiple exposure.
A
Protected Presets
If the current preset is protected, the camera will be unable to measure a
value for preset manual white balance; any attempt to measure white
balance for a protected preset will be accompanied by a flashing 3 in
the control panel and viewfinder.
214 Shooting Settings