Nikon D780 DSLR Camera (Body Only) - Manual (Page 266)

220 Shooting Settings
D Time Restrictions on Measuring Preset Manual White Balance
(Live View)
The time available to measure white balance is that selected for Custom
Setting c4 [
Monitor off delay ] > [ Live view ].
D Measuring Preset Manual White Balance (Live View)
Preset manual white balance cannot be measured while a multiple
exposure is in progress.