Your wording is confusing. Your original post stated that you are trying to use a Set Column action to write a value TO a computed column. That has never and will never be a possibility.
Now you are talking about reading FROM a computed column and I can only assume to write it to a basic column…unless you are trying my to write from a computed column to a computed column…
Maybe we need some clarification on what you are actually trying to do because your statements are contradicting each other.