I have ArcGIS Pro version 3.4.2.
The "Append" tool crashes the application every time I run it.
Does anyone have any insights?
I ran into a similar issue when joining layers and filed a bug https://support.esri.com/en-us/bug/joining-a-csv-file-in-a-flooraware-map-with-a-floorawar-bug-00017...
I suggest you do the same for the append tool.
You can try to deactivate floor awareness and remove the level_id from your the layer you want to append to (if that's an option for you).
Best,
A.