Every once in a while, Screenwriter does not have permissions to write (save) to your computer. The usual solution is to explicitly give Screenwriter permissions to write to your computer by following the steps in this FAQ:
IF THE ABOVE DOESN'T WORK...
Sometimes Screenwriter still doesn't have permission to write to your hard disk even after explicitly granting permissions to it. The solution for that is to do the following:
STEP 1: Remove permissions -- if they are set -- by referencing the following FAQ:
To Remove permissions for Screenwriter, follow the steps in the following link to REMOVE the permissions instead of add them. NOTE: Locate the instructions for the version of MacOS on your computer
STEP 2: Reinstall the software from the following FAQ through Step 15 -- do not re-activate:
LINK: Download and Install the current 64bit Screenwriter 6.7 ALPHA Release
STEP 3: ADD permissions per this permissions FAQ.
That usually does the trick.