“Tankism” isnt a thing, do you mean Marxism-Leninism? In general, you can criticise anything you want, as long as it is based on facts and not on baseless insulting.
“Tankism” from my understanding is an umbrella term for various forms of authoritarianism. Marxism-Leninism is definitely one of those, and so is trotskyism and maoism.
Marxism has strong overlap with “tankism” but the two groups are not equivalent, as there’s a (growing?) number of anti-authoritarian marxists.
You should read the text On Authority by Engels, it is quite short and explains the Marxist stance in this regard.
Well, that text…have you ever tried to read critiques to such? If so, why do you come do a different conclusion, then that Engels renders the term authority useless? In his definition, authority is is justified because it can mean everything. Mutual aid is authority, plants living in symbioses is authority, plants producing oxygen is authority over humans…sure, you can use that definition, but by doing so you simply sneak around a critique against authority by stating it’s inevitable, so the issue can’t be authority itself.
Serious question: which critiques of authority have your read, to which you disagree to such a great amount, that you feel compelled to go with Engels analysis here, that is in short: everything is authority, and because it is inevitable to exist, it invalidates the critique against, without actually engaging with the actual brought up arguments of those that critique authority.