The key point from the community answer is that the SB-910 itself must be set to Master mode for Nikon CLS remote control to work.

If the pop-up flash triggers the SB-600 but the SB-910 does not, check the SB-910 settings first and follow the Master-mode setup in the SB-910 manual (section D, per the answer). Also make sure:

• The SB-600 is in Remote mode
• The group matches what you’re controlling on the commander
• The channel matches on both flashes

If you’re using the SB-910 as commander, the flash should be configured on the flash unit, not just through the camera’s built-in commander settings. In practice, set the SB-910 to Master, then confirm group/channel settings on both units.

So the likely fix is: mount the SB-910, set it explicitly to Master, and match the SB-600’s group and channel to the SB-910’s CLS settings.