I apologize for the delayed reply. It looks like we need to bump up our minimal system requirements. For macOS the minimal requirement has to be increased to OS X 10.10 Yosemite. If you can't update your macOS then you can try using an older version of WireframeSketcher. You can find older versions here: https://wireframesketcher.com/download-archive.html
Buba15 Aug 2019
Version 5.0.4 works fine by me. Can I buy a license for it?