I need to do a membrane pcb for a keyboard or a flexible PCB like a membrane PCB
With custom PCB's like flex or membrane usually the manufacturer will take gerbers, so it doesn't matter what software you use. But the manufacturer will have different requirements than a normal PCB. Pick a manufacturer, then work with them on how to draw up the stackup, minimum trace and contact requirements.

Altium does have the ability to create layers\stackup for flex circuits.