Which meaning of "must" does the Slow spell use?

Question

The word "must" can have two different meanings, both of which are showcased in the crown of madness spell which states:

The charmed target must use its action before moving on each of its turns to make a melee attack...
On your subsequent turns, you must use your action to maintain control over the target, or the spell ends...

Here the first use of "must" means that the charmed target is forced to use its action to make a melee attack, it cannot use its action for anything else.

The second use of "must" means that if you do not use your action, you lose control of the target, it does not force you to use your action maintaining control if you do not wish to do so.

For further discussion on the meanings of "must" there is this Q/A: "When casting a spell with a long casting time, what happens if you don't spend your action on a turn to continue casting?"

The slow spell states:

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.

If a creature starts casting a spell and the slow spell rolls an 11 or higher, is the creature forced to spend their action on their next turn to cast the spell, or can they choose not to and take a different action instead?

Answer 1

Yes, they must complete casting of the spell. It is not ambiguous.

The spell is very clear in it's effect when a caster begins to cast a spell that takes an action and they roll an 11+.

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.

In this case, the caster has attempted to start to cast a spell as an action in Round 1. They then roll an 11+, which triggers the clause of requiring continuing to use their action on the next turn to continue casting.

The only exception here is in the final sentence of:

If it can't, the spell is wasted.

Note that it doesn't say "If it doesn't want to", but the hard line of "If it can't". Only something exterior preventing them from casting at all would remove them from having to continue casting.

Examples preventing a caster would them becoming incapacitated or the spell being cast requiring a verbal component and the caster being caught in an area of silence.