Make sure the it's the latest version. Go here to check: https://flextools.cc/
Make sure SketchUp 2019 was installed correctly by right clicking on the downloaded installer file and clicking on Run as administrator. Then choose the repair option. Also make sure the latest version of SU2019 is installed from sketchup.com/download/all
If after doing those things the error message still shows, contact the Flextools customer support for assistance.