I might have the wrong definition of fascism, but I typically think of it as extreme government overreach and control. Capitalism, however, needs a free market and free-will exchange, both of which government restrict.
If you wanted to argue that capitalism inevitably devolves into fascism you might be able to, but I suspect that any economic system would just as easily devolve to fascism.
In order for capitalism to devolve into fascism, you’ll have to corrupt the government (otherwise there’s no way to override free will exchange). However, what exists in capitalism that makes this easier/provides additional incentives for this? Every system with a government will have powerful people who want to manipulate the government for more power.