One of the effect of slow on a creature is (emphasis mine):
If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn't take effect until the creature's next turn, and the creature must use its action on that turn to complete the spell. If it can't, the spell is wasted.
Does it simply mean the spell fails, and that I don't expend a spell slot and material components since I haven't completed the casting?