There is a new upgrade available to Screenwriter 6.5 that is fully Sierra compatible. We recommend that you consider upgrading (for a fee) to get the new features and future compatibility. Here is more information about the Mac Screenwriter 6.5 upgrade: Mac Screenwriter 6.5 Release Notes
Alternate solution: There are several incompatible versions of Mac Screenwriter 6 (v6.1, v.6.2, and v6.3) and some earlier verisons, but we have a compatible earlier version on our support site. We recommend you download and install Screenwriter v22.214.171.124.
To install the 126.96.36.199 Sierra-compatible version of Movie Magic Screenwriter 6, you MUST throw your current Screenwriter app in the trash BEFORE installing 188.8.131.52. The 184.108.40.206 installer will not install over newer versions of the software, e.g. 6.2 or 6.3.
Once you have trashed the Screenwriter 6 app, you may re-download the software here: