Cannot delete custom timer on Apple Watch

I had this exact same problem. I fixed it by deleting the Timers app and reinstalling it.

If you press the digital crown to view your alphabetical list of apps on the Home Screen, you can swipe left on the Timers app to delete it. Next, you can use iMessage to text yourself this link to the Timers by Apple on the App Store. I opened this message link on my watch, then I was able to reinstall Timers directly to the watch.

Did you already try to force restart the watch by pressing the Digital Crown and the Side button together for about 10sec, until the Apple logo comes back again?

Yes, you can only delete custom timers, default timers can not be removed.

Have a look at the related article:

Set timers on Apple Watch - Apple Support

However, if you want to be able to delete custom timers, you can tell Apple about your wishes for future updates by using the Feedback link. This is the only way to reach them, as Apple does not follow these user to user communities.

Feedback - Apple Watch - Apple

If you have set up restrictions on your iPhone in the settings for Screen Time, for example not allowing changes to your Apple Account, these restrictions may also affect your watch.

Can you move your Control Center icons, as described here?

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copied from: Customize Control Center on Apple Watch - Apple Support

Did you already update to watchOS 11.1?

Sorry for being unclear. A custom timer is a timer that has been set up by the user, the other timers are the ones that come with watchOS, and they can not be deleted.

I created a 8 min timer, which I can delete, but the other ones came preinstalled and can not be deleted.

I'm having the same problem & was only able to delete the bottom 2 times set. The others are pinned & when i got to edit it doesn't allow me to delete the other times that are pinned. I plan to call Apple tech support about it. Maybe they can do a screen share on the watch like they provide on other devices.

Can you move the icons around in the Control Center on your watch?

Any restrictions set up on your iPhone?