In ArcMap and ArcGIS Pro, using Field Calculator with the Python Parser or Expression Type to calculate a new field from Shape.STArea or Shape.STLength returns Error 000539.

Workaround

There are two workarounds:

1. In ArcMap, choose VBScript as the parser. However, VBScript is not an option in ArcGIS Pro.
2. With Python chosen as parser/expression type, instead of double-clicking on Shape.STArea() or Shape.STLength(), change the syntax to !shape.area! or !shape.length! as mentioned in this documentation.

Steps to Reproduce