T
he argument goes something like this. Humans are responsible for their actions; the thing that causes their responsibility is "free will". Philosophers have then gone on to attempt to describe what free will is.
Materialists heard the various attempts to describe free will, and cobbled together an "essence" of the concept. After thinking about it for a while, they decided that the concept of free will was incoherent. And if free will doesn't exist, and free will is the cause of responsibilty, then humans are not responsible for their actions.
Even just logically speaking, this is a complete muddle. First of all, saying that free will causes responsibility can be translated into logical terms as "if responsibility, then free will". But materialists have taken that premise, and converted into "if not free will, then not responsibility". This is not a valid operation. For example, it could be that free will (as conceived) does not exist, but that humans still have responsibility (i.e. responsibility has a different cause). Declaring that one, particular form of free will is incoherent is insufficient to deny that humans are responsible for their actions.
But secondly, and more importantly, the whole formulation of the objection is just backwards. We know that humans beings are responsible for their actions with a very high degree of certitude, much higher than can be granted by the kinds of abstract reasoning that goes on within these philosophical discussions. Furthermore, our experience of free will is perceived as practically a first principle, akin to our experience of color or shape. Extrapolating on its exact nature, how it came to be, how it functions, etc. are interesting and important investigations; but the existence of human responsibility and free will cannot really be doubted. If a particular formulation is incoherent, that may be reason to abandon the formulation, not the thing that we are trying to describe.