I was having similar problems (arcgis pro 2.5.0): when trying to change the labels on a layer symbolized with graduated colors, the labels would attach to the wrong value and/or I would get the "Requested operation could not be completed" message.
I believe the problem arises if the field you are basing the symbology on has Null values. My workaround was to switch to a Unique Values symbology and create a simple Arcade expression to handle the Null values. Now I am able to change the labels and no longer get the message.
It is probably best if you can remove/change Null values in the data/layers instead of writing arcade expressions. In my particular case, that is not an option as my layers are joined to a database view that is constantly refreshed with changing data sources, hence my workaround.