Choose instruments: can't delete lone instrument from the score instruments list
OS: Windows 10 (10.0), Arch.: x86_64, MuseScore version (64-bit): 184.108.40.2061380634, revision: 10dee08
Open the New Score Wizard and go to "Choose instruments." Create a single instrument then try to delete it.
Result: You can't. You have to press "Back" then "Next".