You should be using the (saved-games:) macro to test if a saved game exists. You're testing against a variable called $Saves. If you didn't previously set this variable to (saved-games:) then that would cause the error.
You almost certainly didn't declare the $Saves variable anywhere, since undeclared variables in harlowe are treated as the number 0, hence the error message.
Example using the (saved-games:) macro:
(if: (saved-games:) contains "Slot A")[..,
Example using a variable:
(set: $Saves to (saved-games:).
(if: $Saves contains "Slot A")[...
Note. If you use the latter method, you'll need to include the (set:) just before the check as shown; since harlowe doesn't pass objects by reference, but rather by copying/cloning them, setting the variable once won't work because changes to the save data won't be available to the variable.
All in all, you probably should go with the first method.