The Append geoprocessing tool returns the error, "ERROR 999999: Something unexpected caused the tool to fail" when running the Python script. When the preserve GlobalIds environment setting is equal to True, the Append tool fails to append features with a GlobalID field to the target feature class.
This is a duplicate of BUG-000101690. It is documented that the Preserve Global IDs environment only applies to enterprise geodatabase data used with the Append tool. The message will be improved to indicate this. https://pro.arcgis.com/en/pro-app/tool-reference/environment-settings/preserve-globalids.htm