I am designing a Scissor lifter and I am operating my hydraulics using solenoid valve but I want my system to be ready for every possibility so what should be the backup system if my solenoid valve stops working
'Every possibility' is not a specification. It's quite difficult to allow for meteor strike for instance.
You need to decide what failure modes are permissible, and what not permissible. With a hydraulic lifter, moving is more dangerous than stopped, at least in my book. That means you should make sure it's always possible to stop the platform.
You need to decide what component failures are likely, and what unlikely. Perhaps a valve sticking open is something you want to guard against.
Stopping the lift even in the event of an open valve implies having two valves in series wherever you might use one, the likelyhood of them both sticking open is then much smaller than the failure of one.
If they are driven by the same power supply, this could be a point of common mode failure. If you think the supply sticking 'on' is a possibility you want to guard against, through a failed short output device for instance, then you need to duplicate supplies.
You can make a 'majority voting' valve with 3 parallel paths each of 2 valves in series, driven with 3 skew circuits, so (ab, bc, ca). That way, if 0 or 1 of abc is driven, the flow will be off, if 2 or 3 are driven then the flow will be on. Do you want such complexity for protecting a tiny part of your system?
For a full manual overide, a manual valve in series with the solenoid will stop the flow if it is stuck on, and a manual valve in parallel with both will operate the lift if the valve is stuck off.