Product and version
FileMaker Pro 15.0.2
OS and version
OS X 10.11.6
iMac Retina 5K 27''
Unexpectedly, the GetFieldName function, invoked by a calculation field, does not allow to store the calculation result, if based on a related field.
How to replicate
Create a calc field on "Table1" with this formula : GetFieldName ( Table2::DATA )
Using storage option button, try to uncheck "Do not store calculation results -- relaculate when needed"
You generally can convert this field on a auto entered field with the same formula.
Since GetFieldName allow you to refer an unrelated field (because it does not return data but structure information), this restriction is unexpected.