The one-word payback is a noun and an adjective.1 It does not function as a verb. The corresponding verb is pay back—two words.2 So when you get payback from someone, you make them pay back what they owe.
In English, we have many compound words similarly derived from phrasal verbs, and the one-word forms almost never gain acceptance as verbs. For a few other examples, see blowup–blow up, runaway–run away, and rollout–roll out. Of course, the one-word forms are inevitably used as verbs in informal writing, but the one-word/two-word distinction is generally observed in edited writing.
This was a long-nosed lie, and I knew that payback was coming. [Atlantic]
And one could perhaps expect a little payback for all the Polish builder jokes. [Guardian]
Changing payback percentages on slot machines is not done routinely. [Joliet Herald-News]
The payback period is still long – around 20 years. [Montreal Gazette]
Pay back (verb)
If insurance costs go up too much there won’t be any money left to pay back the bank. [Stuff.co.nz]
Bonds are a promise to pay back your loan of money on a pre-set schedule. [Forbes]