Currently, the Workflow Designer requires a 32-bit version of Visio (2010, 2013 or 2016). Most people (I would guess) have the 64-bit version installed. They need to uninstall that and install the 32-bit version.
Please start supporting the 64-bit editions.
Also, if you could add support for Visio 2019 (the latest version), that would be even better.