Can't Add My ZEN26 On/Off Switch to Home Assistant

Exclusion is just a soft reset. A device doesn't need to be included first to be able to excluded:

In Home Assistant: open Settings > Devices and Services > Z-Wave JS > Configure
Click on Remove Device
Click on Start Exclusion
Tap the lower paddle of the switch 3 times very quickly to finalize the exclusion process
The device will now be removed, but that won’t show until you restart Home Assistant
Run a Heal Network so all the other nodes learn about its removal

After exclusion is confirmed, try adding the device back to Home Assistant again.

If that didn't work, here are some troubleshoots we find helpful:

Try the exclusion process again! It's possible the exclusion commands from the first run weren't registered correctly.
Try the exclusion process again, only this time instead of tapping 3 times, tap the paddle 6 or 7 times at least as quickly as possible. This can help make sure the commands go through.
If you have a Z-Wave signal repeater available (switch, relay, smart plug, dedicated repeater - any Z-Wave device that doesn't rely on battery power), we'd recommend moving it between the switch and hub and then healing the network. This can help in case there is an interference or range issue preventing stable communication between the two nodes. This can be especially helpful if your switch box is made of metal (since metal shortens and interferes with Z-Wave signals).
If possible, try bringing the hub within direct range of the switch if you were at a large distance the first time. If you have a hand-held secondary controller, this would be the time to bring it out!
And if you find that your switch is connected but it's not responded to remote commands, check out these tips for troubleshooting connectivity.

If you've tried all the above with no luck, let us know! Our team is ready to help.