Edit coach training plan activity

And a good day to you, sirs! I'd want to tinker with an activity part of my Garmin Coach training plan, mainly to make warmup last until lap button press instead of just 2:00 (I want to know where I'll be when I'll start counting the miles, to better plan my course).

Opening the workout on my Instinct Crossover Amoled, I have an edit option, which sends a notification to my phone opening the Connect app into editing my activity. However, when I try to save it, an error always occurs. Is this a bug? Should I be able to apply modifications to such an activity and the app is not working as intended? Or instead I shouldn't be able to tinker with what my coach tells me to do at all, and being given an option to do it, albeit a non working one, is the real mistake?