The reason causes the relevant shortcuts failure is the Apple system defects in the system upgrade of iOS or WatchOS.
Here are the tips to solve the problems:
(Please note the following steps are reprinted from Apple's support forums. We still recommend that you seek support from Apple's official customer service first.)
1- As for the Siri on the Apple Watch doesn't recognize shortcuts.
Please check if the shortcuts are the same on Apple Watch and iPhone. If they are not the same, it might be caused by iCloud or the information transfer mechanism of Siri Shortcuts.
If this phenomenon has a high frequency, please try to pair the Apple Watch with iPhone again.
2- Some issues appear after iOS has a major version upgraded, such as the version to iOS 14.0 last year and iOS 15.0 this year.
As for Issues like Shortcuts do not execute, automation doesn't work, prompt to ask you to allow (null) to visit your data and so on, we can refer to the below methods.
A- Reboot iPhone and Apple Watch
B- Update to the latest system
C- Pair the Apple Watch with iPhone again
D- Delete the shortcut APP on your cellphone > reinstall the shortcut APP > pair the Apple Watch with iPhone again