Canon EOS REBEL T7i Instruction Manual page 166

B: Matching the Light SourceN
If the exposure obtained in step 1 differs greatly from the standard

exposure, a correct white balance may not be obtained.
In step 3, the following images cannot be selected: Images captured with

the Picture Style set to [Monochrome] (p.156), images shot with a
Creative filter, images processed with a Creative filter after shooting,
cropped images, and images shot with another camera.
The personal white balance registered with EOS Utility (EOS software,
p.474) will be registered under [O]. If you perform step 3, the data for the
registered personal white balance will be erased.
166
Select [O (Custom)].
4
Press the <WB> button.

Select [O (Custom)], then press

<0>.