Because of this issue, thi plugin currently doesn't work in MuseScore4.
Musescore plugin to add saxophone fingerings to your charts.
Created because of this thread
install the Saxy font (TTF, OTF)
Select your part and run the appropriate plugin.
Note that you will probably need to resize the fingerings to your need. To do that, right-click one -> select similar -> set the font size in the inspector.