Learn Turkish: İyi bir plan her projenin olmazsa olmazıdır. - A good plan is an absolute must for every project.

It literally translates to "if it doesn't happen, it doesn't happen" (or "if it isn't, it isn't"). Olmaz is the negative aorist (broad tense) of olmak (to be/happen), and olmazsa is its conditional form ("if it doesn't happen"). In Turkish, this entire phrase has been frozen into a noun meaning "an absolute must" or "an indispensable thing."

Why is there an -ı suffix attached to olmazsa olmaz?

Because it is part of a genitive-possessive chain: her projenin olmazsa olmazı (every project's absolute must). Even though olmazsa olmaz is originally made of verbs, it acts completely like a regular noun here. It takes the 3rd-person possessive suffix -ı to link back to the genitive noun projenin.

What is the purpose of the -dır at the very end of the sentence?

The -dır suffix acts as the "to be" verb here. Since the subject is İyi bir plan (A good plan), the sentence needs a verb to say what it is. Adding -dır to the end of the noun phrase makes it a complete statement of fact: "is the absolute must."

Can I use olmazsa olmaz as an adjective too?