Therefore, according to my reading, there are two different ways of interpreting this phrasing:
"You must pass your action every turn by casting the spell. [and if you don’t, the spell fails]"
This is the simplest thing: to continue casting the spell, you must continue to use your Action; If you do not, the spell fails. There is not much more to say about this.
"You must pass your action every turn by casting the spell. [literally. The spell physically compels you to continue casting until the spell is complete; you cannot choose to use your Action to do something else until the spell is complete or until your Concentration is broken]"
This is, in my opinion, a valid interpretation of the literal language as well.
[RegardlessofhowweintendtointeractwithotherrulesofemphasisIhaveseveralreasonstothinkthatthissecondinterpretationis[withoutconsideringintentorhowthisinteractswithotherrulesIshouldstressIhavevariousreasonsforfeelingthissecondinterpretationis[SinconsiderarlaintenciónocómoestointeractúaconotrasreglasdeboenfatizarTengovariasrazonesparasentirqueestasegundainterpretaciónes[withoutconsideringintentorhowthisinteractswithotherrulesIshouldstressIhavevariousreasonsforfeelingthissecondinterpretationisdo not how the rule should be interpreted, but that is a different answer for a different question.]
Basically, we have two meanings of the word & # 39; must & # 39 ;: in one case, indicates obligation: you should Keep using your action or the spell will fail. In the other case, it indicates compulsion: you. should keep using your action; he is not allowed to choose otherwise.
However, regardless of the interpretation, the interpretation of the user who cited, paraphrased below, I think is very unlikely:
"You must pass your action every turn by casting the spell. [but if you can’t or choose not to, the casting of the spell will continue or suspend]"
Only from the design point of view, it does not make much sense for an effect to say that it "must" do something to cause an effect, but it implies that if it does not do that, the effect will occur anyway. Consider the language of a spell as Animate dead:
On higher levels. When you cast this spell using a level 4 or higher spell slot, you animate or reaffirm control over two additional undead creatures for each level of the slot above the 3rd. Each of the creatures must come from a different corpse or pile of bones.
–Animate dead, Player's manual, p. 212
It does not expressly tell us what happens if the creatures come from the same stack, but it is still clear from the context that the effect does not work if this is the case. Other effects in this game are written very similarly: if a condition is specified for what the pitcher "must" do, those conditions must be met. The absence of an explicitly stated consequence for not doing so is not necessary for such a consequence to exist.
Therefore, regardless of the interpretation I quoted is correct, the user who quoted is almost wrong. If the spell caster does not use his Action in a given turn to continue casting his spell, for whatever reason, the spell will fail. I have already indicated in my answer to the linked question that under certain circumstances (the Surprised condition, for example) Personally, at my table, I would ignore this rule, but as a general principle of the written rules, this is how the rule works.