I'd like to report a bug: Spatial Analyst Tools - Zonal Statistics as Table - ERROR 999999 - Zone field parameter - field name length issue: The field name must be short (as in the number of characters in the name of the attribute field), or the tool will fail. Not sure what the threshold is, but I tested and please fix, or notate in the tool help.
Solution Solved: https://support.esri.com/en/technical-article/000012343
...Can you please update the tool help for Zonal Statistics as Table and also Zonal Statistics?
Which contains lots of information on naming conventions gleaned over the years.
In short... keep it short and dump the spaces.
But read the whole blog