The card reads “All creatures get -4/-4 until end of turn and all Vanguards lose 4 shield”. However, if I don’t control any creatures I cannot activate the spell. Both me and my opponent have to have creatures on the battlefield to be able to use the card even though it’s not a requirement on the text.
On this first screenshot I had no creatures on the battlefield and my spell was disabled
On this other screenshot I had one creature out and now the spell is enabled and ready to use