My version is ArcGIS Pro 2.3.2
I had same error message "Requested operation could not be completed" when attempted to change symbology from Single Symbol to "Unique Values" (this feature class has more than 100 unique values). I also noticed followings.
1. Failed to open its attribute table (getting message something like "Could not load the data, General error...."
2. I was able to view same feature class in ArcCatalog 10.5.1
3. I was able to view same feature class in ArcGIS Desktop 10.5.1. and able to change symbology to Unique Values
After attempting to fix this issue with my limited knowledge (refresh, remove feature class from Contents Pane and add it again, close the project and reopen...), my application suddenly closed without any warnings. I then open it and the issue was fixed!!
I have no idea as to why or how this happened but thought to share what I experienced.