Nope, it's not valid Whitespace!
Try it online.
All characters that aren't a space, tab, or newline are ignore in Whitespace. So this program is actually SNSSS (where S is space, and N is newline). The first three (SNS) is the command to duplicate the value at the top of the stack. Since the stack is still empty, it gives the error:
wspace: user error (Can't do Dup)
I tried other errors, but this seems to be the shortest. Most errors where it tries to use a value on the stack which isn't present are similar, but longer. Here is a list of all possible errors (I could find) on TIO (for the first one it requires a character input, which it tries to read and print as number, so I've excluded that one - Try it online.):
wspace: Prelude.read: no parse wspace: user error (Can't do Dup) wspace: user error (Can't do Swap) wspace: Prelude.!!: index too large wspace: user error (Can't do Store) wspace: user error (Can't do Return) wspace: user error (Can't do Discard) wspace: user error (Can't do Slide 0) wspace: user error (Can't do ReadNum) wspace: user error (Can't do Retrieve) wspace: user error (Can't do ReadChar) wspace: <stdin>: hGetChar: end of file wspace: <stdin>: hGetLine: end of file wspace: user error (Can't do OutputNum) wspace: Prelude.chr: bad argument: (-1) wspace: user error (Can't do Infix Plus) wspace: user error (Can't do OutputChar) wspace: user error (Undefined label ( )) wspace: user error (Can't do Infix Minus) wspace: user error (Can't do Infix Times) wspace: user error (Can't do If Zero " ") wspace: user error (Can't do Infix Divide) wspace: user error (Can't do Infix Modulo) wspace: user error (Can't do If Negative " ") wspace: Input.hs:(108,5)-(109,51): Non-exhaustive patterns in function parseNum' wspace: Unrecognised input\nCallStack (from HasCallStack):\n error, called at Input.hs:103:11 in main:Input wspace: Stack space overflow: current size 33624 bytes.\nwspace: Relink with -rtsopts and use `+RTS -Ksize -RTS' to increase it.
NOTE: Whitespace compilers have their own implementations for error messages. All these errors above are on TIO. If I use the online Whitespace compiler vii5ard instead, and use the same program at the top, it will give this error instead:
ERROR: Runtime Error: Stack underflow
So using the vii5ard online Whitespace compiler I could lower my byte-score to:
Unexpected EOF!
Which is the 'program' S (a single space), resulting in the error:
Unexpected EOF
(Which would result in wspace: Unrecognised input\nCallStack (from HasCallStack):\n error, called at Input.hs:103:11 in main:Input on TIO).
__main__.CodeException: Raised an &rror." my error or is this:"Raised an &rror." \$\endgroup\$