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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?
Searching Merriam Webster for the applicable definition of “waste”, I found these two that seem most relevant:
a : to spend or use carelessly : SQUANDER
waste valuable resources
b : to allow to be used inefficiently or become dissipated
If the spell slot was not lost, it wouldn’t make sense to say the spell was “wasted”. “Wasted” indicates that something is lost, squandered. In this case, the resource used to cast the spell is wasted - a spell slot (and components, if applicable).
