Photos Library/ Editing/ Red Eye.

Have you seen this page in the Photos User Guide? Remove red-eye from photos in Photos on Mac – Apple Support (UK)

The procedure is working well, if Photos could automatically detect a red eye. Otherwise you may need to position the pointer over the red iris and adjust the size.

1. In the Photos app  on your Mac, double-click a photo, then click Edit in the toolbar.
2. Click Adjust in the toolbar.
3. In the Adjust pane, click the arrow  next to Red-eye.
4. Click Auto.
5. Photos removes any red-eye that it detects in faces in the photo.
6. If red-eye still appears, drag the Size slider until the circle is the same size as the red area in the person’s eyes.
7. Position the pointer over the red pupil and click. 
8. Repeat this step for all other red eyes in the photo.