Indispensable
Essential
noun, な adjective
It's not possible to lack. It is indispensable and essential, so don't lack it!
ふかけつ
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
ビーチに行く時は、日焼け止めが不可欠です。
When I go to the beach, sunscreen is essential.
日本語がペラペラになるのに不可欠なことは何だと思いますか？
What do you think would be essential to becoming fluent in Japanese?
美味しいベーコンは、サクサク仕事を進めるのに不可欠です。
Tasty bacon is indispensable in promoting efficient work.