CS1617 Invalid option 'preview' for /langversion. Use '/langversion:?' to list supported values

[andydis]

Error when compiling:-
CS1617 Invalid option 'preview' for /langversion. Use '/langversion:?' to list supported values

windows 10 1903
Visual studio 2017

Tried:
installing all the .net SDK's / developer packs
changing the projects target framework to 4.0 , 4.5 , 4.6.1 , 4.6.2 all same error

1>------ Build started: Project: SharpLocker, Configuration: Debug Any CPU ------
1>CSC : error CS1617: Invalid option 'preview' for /langversion. Use '/langversion:?' to list supported values.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========