void TestFunction() returns void.

void * temp_void(); is the wrong signature for TestFunction as the return type here is void*.

The correct return type would be void because that is exactly what TestFunction returns.

So your example should be:
Code:
#include <acknex.h>

void TestFunction()
{
	error("TestFunction was called");
}

void CallFunc(void * func)
{
	void temp_void();
	temp_void = func;
	
 	if(temp_void) { temp_void(); }	
}

void main()
{
	CallFunc(TestFunction);
}

Always learn from history, to be sure you make the same mistakes again...