It's like an itch you can't scratch
Selling crack is something you can’t stop doing easily, earning that much money becomes addictive.
The line is also an allusion to drug fiends that consume crack cocaine. They are often portrayed as scratching a non-existent itch.
Pusha uses that imagery to propel the song into the next line in which he makes clear that he is, in fact, using the term as an idiom.
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