For the last two days I can't log my weight in the app cause the weight screen is blank and there's no possibility to log it. Why is that and what can be done to fix this?
Nothing happens when you select "Weight" at the top of your My Day screen?
I would try uninstalling, then installing the app. If still no joy, contact Garmin Support.
Meanwhile, if you need to enter your weight, you can do it from Garmin Connect Web - Health Stats > Weight
