Thank you for the information. I'm not sure what the problem may be, and I've not been able to duplicate your problem. I used the exact verbiage your quoted in your original post and it created the timed event right away. Of course, since it is already past 1:00pm, it created it for tomorrow. I also tried the message you quoted in your second response and it created a reminder for one hour from now.
Generally, Siri will only respond with a problem message if there is difficulty connecting to the internet. I have an Apple Watch Series 6 with watchOS v7.3, and an iPhone 12 Pro with iOS 14.4.
At this point, if resetting the Watch did not help, I would suggest trying to turn off Siri, then power the iPhone off and back on again, then turn Siri back on again and test it. If that does not help, then use the Contact Support link at the top of the page here to contact Apple Support for assistance. I cannot think of anything else to do.