The phrase "could have been avoided" refers to the past, but not "could be avoided". For example, referring to a past event, "the accident could have been avoided through proper maintenance procedures". In the future, "accidents could be avoided".

they both can refer to the past with some difference in meaning. 'duels could be avoided' means that it was possible in past to avoid duels, it refers to a 'state', 'general truth'. 'the duel could have been avoided' refers to some duel that took place but could have been avoided. Study this: https://learnenglish.britishcouncil.org/en/english-grammar/verbs/modal-verbs/can-could-and-could-have

'Could be avoided' can also be used in the present and future contexts. You might talk about something you are doing that could be avoided, whilst 'could have been avoided' is only a past tense.

thank You ALL a lot for giving so many attention to my question!!!

So let that be a lesson to me! I jumped straight for the future use of "could be avoided" and completely forgot about that particular past use. It could not be used for the case of the Titanic: it would only have a future meaning, and would not mean 1912 !