macOS says "EasyKOT is not open anymore"?
Your sales are safe. Orders and bills live in the register's own database, not in the process that is stuck. What happened is that an old EasyKOT process is still running with no window, and macOS keeps trying to bring that one forward instead of opening the app. There are two ways out: the easy one, and the one for right now.
Why this message appears
Now and then, on the restart after an update, the old EasyKOT gets stuck while closing, and the new one gets stuck with it before it can open a window. macOS then believes EasyKOT is already open, tries to bring it forward, finds no window, and says: The application "EasyKOT" is not open anymore.
The fix, in order.
Easiest: restart the Mac
Apple menu → Restart. The leftover process is gone after a restart and EasyKOT opens from the Dock as usual. If you are mid-service, do step 2 now and restart after closing.
Need it now? Open a fresh copy from Terminal
First make sure EasyKOT really is not open anywhere, including your other screen. Then open Terminal (type Terminal in Spotlight) and run: open -n /Applications/EasyKOT.app. macOS then ignores the old process and starts a new register.
No window? Look at your other screen
With a second screen attached, the EasyKOT window can open over there. A dot under EasyKOT in the Dock means it is running: find the window in Mission Control (F3, or swipe up with three fingers) and drag it onto your screen.
Optional: end the leftover process
Open Activity Monitor and search for desktop, which is the name of the program inside the EasyKOT app. Select the old desktop process with no window and press Force Quit. If it will not go, leave it: a restart removes it, and step 2 keeps you selling until then.
One thing to watch. Run open -n only when EasyKOT really is not open. If the register is already running and you run it anyway, you get two registers on one laptop working on the same database. If that happens, close one of the windows.
Straight answers
Did I lose any sales or bills?
Why did it happen right after an update?
Does this happen on Windows?
What if it keeps happening?
Run it 3 months, then pay for it.
Three months free, full product, no card. Made in India, for the counter: built inside a running restaurant.