Learn Polish: Cel projektu musi być mierzalny i osiągalny. - The goal of the project must be measurable and attainable.

It is formed from the imperfective verb mierzyć (to measure). We take the root mierz- and add the suffix -alny. This suffix acts just like the English "-able" or "-ible", creating an adjective that means "capable of being measured".

Why is osiągalny derived from the imperfective verb osiągać instead of the perfective osiągnąć?

Adjectives of potentiality ending in -alny and -elny are almost exclusively built from imperfective verbs. Even though completing a goal feels perfective, the general capability of being achieved is treated as an ongoing, permanent trait, so the imperfective base is required.

Why do we use projektu here instead of the dictionary form projekt?

Projektu is in the genitive case. In Polish, when you place two nouns together to show possession or association (translating to "the goal of the project"), the second noun must take the genitive case.

Can osiągalny be used to describe people as well as abstract goals?

Yes, but the meaning changes slightly. When applied to a project or a goal, osiągalny means "attainable" or "achievable". When applied to a person, it usually means they are "reachable" or "available" to be contacted, such as being reachable by phone.