What to do about “[Shape_STLength__]” Errors?
Database error: The field is not nullable. [Shape_STLength__]. The geometry is not Z-aware.
That field shouldn’t be there, you can safely remove it. I wrote a quick script to do so here: https://gist.github.com/morehavoc/3e86f535bf204b040ee2
Now for the full story…
For me the field was in a feature class that I had created via a Python script using another feature class as a template; that was the issue. Esri, being so helpful, decided to convert the Shape.STLength() field into Shape_STLength__ and to top it off, make it required as well. The field Shape_STLength__ has no use, other than to cause trouble. The Shape.STLength() field is not actually a real field, it is a alias created by Esri to get the length of the geometry, handy right!
Once I figured out that was the issue, I was able to remove all of the Shape_STLength__ and Shape_STArea__ fields using a python script (making sure to shut down my services first) and all was well once again.
You can check out the python script here: https://gist.github.com/morehavoc/3e86f535bf204b040ee2