A human being is a deciding being.

Viktor Frankl (1905–1997) Austrian neurologist, psychiatrist, philosopher, writer

(Paraphrase)

Widely attributed to Frankl, but not found, as such, in his writing. The Frankl center in Vienna previously had the following posted on one of its pages:

A human being is a deciding being. And what does he decide? What he will become the next moment.

This was labeled "Freely translated Viktor E. Frankl."

The phrase is sometimes cited to Frankl's most famous book, Man's Search for Meaning, but is not found there in its different editions (1946; https://archive.org/details/frankl-viktor-mans-search-for-meaning-1963/).

That said, as the "freely translated" indicates, this may be a paraphrased translation from the original German, separate from the formal English translations above. And it is in keeping with Frankl's overall existentialist/Logotherapy theories, e.g., in his The Doctor and the Soul (1952) [tr. Winston (1955)]:

Man, as a person, is not a factual, but an optional being; he exists as each his own possibility, for or against which he can choose.

(Note: this appears to be another case of a separate English translation at the Frankl Center website from what is actually in the English edition of the book.)

A clue to the original quotation above may come from this passage that is in The Doctor and the Soul:

Existential philosophy deserves the credit for having proclaimed the existence of man as a form of being sui generis. Thus Jaspers calls the being of man a “deciding” being, not something that simply “is” but something that first decides “what it is.”

This refers to German-Swiss psychiatrist/philosopher Karl Jaspers (1883–1969). Jaspers describes human existence (Existenz) as a deciding being (entscheidendes Sein), that a human is not a fixed object which simply is, but an open possibility defining and creating what it is through active choices.

The original "free translation," then, may be a reframing or paraphrase of Frankl's reference to Jaspers, reshaped into a statement of his own.