If you are getting a message on your Apple Watch informing you that "the folder could no longer be found after the latest sync", try the following troubleshooting steps:

First, within Seconds for iPhone go to Settings > Apple Watch > Sync Timers Now.

If that does not resolve the problem try the following. One thing that has been reported to help is going into the Watch app on the iPhone and navigating to Reset > Reset Sync.

Once that is done, restart the iPhone.