Correct spelling, explanation: the word finally is an adverb, therefore it’s a derivative of the adjective final. Most adverbs are formed by adding the suffix -ly to the end of the basic word. The adjective ends with an -l, so the additional l must be present because of the adverbial -ly suffix.

Definition of finally:
adverb, at the end of some time, eventually, as the last point
I finally reached an end.
The criminal was finally arrested.
Finally, I wanted to thank my mother and father…

Collocations with finally:
Some most commonly used collocations include: finally manage (to do something), finally find, and finally succeed in something.
I finally managed to finish my work!
Jane finally found her keys.
My dad finally succeeded in fixing his computer.

Incorrect spelling, explanation: since adverbs in the English language are derivatives of adjectives, the word finally must accord with its root, which is the word final. It originates from Old French final and from Late Latin finalis (meaning of or pertaining to an end, concluding, final). When forming an adverb, one must add the suffix -ly to a basic word. That’s why the correct spelling is finally and not finaly.

Incorrect spelling

Incorrect spelling

Incorrect spelling