This is the 977th most frequent Japanese word.
成立
"成立" means "establishment" or "completion."
Here, "成立" refers to the successful establishment or conclusion of a contract or agreement.
契約が成立しました。
The contract has been concluded.
Here, "成立" is used to express the condition of something being tenable or logically sustainable.
この条件では議論は成立しません。
Under these conditions, the discussion cannot be sustained.
In this context, "成立" means something becoming valid, feasible, or viable.