Most probably due to installing and uninstalling various Qt versions, QtDS promoted to link itself to a valid Qt installation though it used to be linked in the past.

The prompt opens a dialog which shows the standard Qt installation folder location. For some reasons this folder path is marked red and the "Link to Qt" button is disabled. Opening this folder and selecting it via "Choose..." does not change the situation.

Hint help window which is shown after keeping the mouse on the red text brings a message which claims the Qt installation folder location invalid and does not give helpful hints what is not OK.

See attached screenshots.