Learn Russian: Всегда знаешь, когда уместно сказать откровенный комплимент. - You always know when it is appropriate to give a frank compliment.

This is a generalized-personal clause (обобщённо-личное предложение). By dropping the pronoun ты and just using the second-person singular verb знаешь, the sentence makes a broad statement about what "one" knows or what people in general know, rather than addressing a specific person. It functions exactly like the generic "you" in English.

In English we "give" or "pay" a compliment. Why does this sentence use сказать?

Russian uses different verbs for compliments. The standard collocations are сказать комплимент (literally "to say a compliment") and делать комплимент (literally "to do/make a compliment"). You cannot translate "give" or "pay" word-for-word here.

Since the sentence starts with Всегда (always), shouldn't the verb сказать be imperfective (говорить)?

No, because the adverb всегда only modifies the main verb знаешь (you always know). It does not apply to the infinitive сказать. The subordinate clause (когда уместно сказать...) refers to executing a single, completed act of giving a compliment at that specific right moment, which is why the perfective aspect is used.

What is the grammatical role of уместно in this sentence?