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Which of the two is better? I am afraid that neither fits here.

Negotiation is not common/usual/ just an everyday thing, but it is also crucial for an efficient and satisfactory life.

Thank you in advance
Comments  
Sorry, perhaps it will be clearer now:

Not only is the negotiation common in everyday life but it is also crucial for an efficient and satisfactory life.
You don't need the the before negotiation, otherwise ok. Possibly the word compromise would fit the sentence better.
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AntonijaWhich of the two is better? I am afraid that neither fits here.
Negotiation is not common/usual/ just an everyday thing, but it is also crucial for an efficient and satisfactory life.

"... is not common/usual, but it is also ..." does not work.

"... is not just an everyday thing, but it is also ..." is structurally possible, but here the resulting meaning does not seem terribly coherent. It also completely changes the sense from "negotiation is not common" to "negotiation is common".
Antonija
Not only is the negotiation common in everyday life but it is also crucial for an efficient and satisfactory life.

This makes some kind of sense, but be clear that you are saying that negotation is common. Do you actually need the "Not only ... but" structure? I'm not completely convinced that it's justified here. Could you just say " ... is common ... and is crucial ..."?

If you are referring to a particular type or instance of negotiation that has previously been mentioned, then "the negotiation" is possible. If you are talking about negotiation generally (which on the face of it seems more likely) then you do not need "the".
Of course I could use ..is common..and ...is crucial.., but I need to stress that common is understatement in this case.
Antonija
Of course I could use ..is common..and ...is crucial.., but I need to stress that common is understatement in this case.

Yes, "not only is it X, but it is also Y" often suggests that Y somehow trumps X. My quibble here is that negotiation being common in everyday life seems a consequence, or perhaps parallel, of the second statement, not an understatement of it.
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OK, thank you Mr Wordy. You are probably right.