TermExcelerator: "Target selection cannot be empty" error


I have version of the TermExcelerator and use it with Studio 2019 SR CU4.I decided to try the "Add" functionality in the Termbase Viewer window. I followed the instrucitons from there: https://multifarious.filkin.com/2016/03/02/committing-the-cardinal-sin/. However, as soon as a click the Add button, there is an error: "Target selection cannot be empty". As a result, I cannot add any terms.

Could anyone let me know, if I am doing something wrong maybe? Or if not - how to troubleshoot this.

Thanks, Lyudmila