Take a look at the item again:
If his friends walked into the bookstore
(A) Boris had a stack of car
magazines as camouflage, (B)
but he was really reading Better Homes and Gardens, studying
decorating ideas (C)
to spruce up his first apartment.
You wanted to do this:
If his friends walked into the bookstore Boris had a stack of car magazines
as camouflage but he was really reading
Better Homes and Gardens, studying decorating ideas to spruce up
his first apartment.
Removing the comma before the coordinating conjunction but creates an error. This but connects two main clauses, so you do indeed need that comma.
You might want to review the rules.
Go back to the sentence to try again.