Indispensable
Essential
noun, な adjective
It's not possible to lack. It is indispensable and essential, so don't lack it!
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.
メガネは私の生活に不可欠です。
Glasses are an integral part of my life.
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?
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.