What watch do you exactly mean? Very different watches are said to run Harmony OS. When you can run Android 8 apps on the watch, you can run Juggluco on it. The functionality will be restricted. You can't use the watch face or the complications. You can run the app itself and use floating glucose (https://www.juggluco.nl/Jugglucohelp/floatingconfig.html).
Because you can't make use of Google Play services, you have to configure the connection between the phone and the watch yourself (https://www.juggluco.nl/Jugglucohelp/connectionoverview.html).
The menu to do that on the WearOS version of Juggluco isn't shown; you have to blindly press two times on the screen to get into it, or you can first install the Android Wear version and after configuring the connection with the phone, update to the WearOS version.
Because the Harmony OS watches that can run Android apps, aren't cheap, I don't see why you would buy such a watch when it to run Juggluco.

Thank you for your answer. I was thinking to buy this watch: https://consumer.huawei.com/levant/wearables/watch-fit4-pro/
Is an watch fit4 pro, because of the rectangular shape but I want to run juggluco on it. Or, what is the best watch to buy this days? I saw that wearos has improved bluetooth connection on the latest update

Juggluco will not run on it. You can see that already when looking at the battery life. A watch running Android apps will not run for 7 days on a 400 mAh battery.
It has to be the Harmony OS interface without Android underneath: https://packetai.com/running-android-apps-on-harmonyos-an-evolving-story

WearOS updates don't improve Bluetooth connectivity.