G'day Margaret. Please try this:
Tap on the watch TIMER icon
then scroll to the bottom and press EDIT.
Now every timer has either JUST a yellow PIN, or (for your own custom timers) you will see:
BOTH the yellow Pin AND the red Delete options.
If you don't see any timers with the RED delete option, then you have not yet created a custom timer.
Try pressing the + symbol at the end of the timers list to create a new timer, then press EDIT to see the RED delete option (but only on that one newly created timer).
The timers you CANNOT delete are: 1,3,5,10,15,30 min, plus 1 and 2 hour. These are always at the top, and custom timers are below them. e.g. I have 2,7,45 min plus 1h15m, and 1h30m below the Apple timers.
When I look at the ones *I* have created (using the EDIT option at the bottom), ONLY the 5 custom timers have the delete option on each one of them.
I'm assuming your watch is capable of running WatchOS 11.1 or later, and your iPhone is also capable of installing it.