Quiere usar WHERE EXISTS
:
UPDATE MusicianInstrument mi
SET mi.instrumentName = 'Tambourine'
, mi.levelOfExpertise = 'Adequate'
WHERE EXISTS ( SELECT 1 FROM Musician m
WHERE m.musicianNo = mi.musicianNo
AND m.musicianName = 'Helen Partou');
En una nota al margen, dado que está utilizando Oracle, recomendaría no usar CamelCase o headlessCamelCase para nombres de objetos.