Hw do I fix this hole in the hollow wall is not drywallor plaster ?

You will need to put a thin piece of wood or mdf inside the hole and the secure it with screws on either side of the hole. This way you will have something as a base for spackle. Spackle it, let it dry and sand. You may need a few coats until it is smooth and level with the wall. Then you can paint it to match the wall.

The simplest solution is also the best: cutting a patch from another piece of drywall and securing it with wood backing strips and dry wall screws. Once the patch is in place, tape and "mud" (apply joint compound) over the seams, just like when installing new drywall. If you finish the patch carefully, it will not be visible.

You can patch this just as you would drywall. The most difficult part is going to be in matching the finish. First, cut the hole out evenly and run 2 drywall screw barely into the wall (1 above the hole and 1 below). Cut a 2x4 3-4 inches longer than the hole. Turn and insert the 2x4 into the wall and straighten up so that the screws can be screwed into it. Cut a piece of drywall the size of the hole. Place in the hole and screw into the 2x4. Then you can tape the edges and apply sheetrock mud. Smooth (I use a damp sponge instead of sanding) and repeat over 3 applications until it is smooth. Then you can paint and finish the area.

Watch this YouTube, it explains how to do it (3 ways), even though yours is not dry wall, it is the same principal:

https://video.search.yahoo.com/yhs/search?fr=yhs-vz-vz&ei=UTF-8&hsimp=yhs-vz&hspart=vz&p=patch+a+hole+in+drywall+with+no+stud&type=verizon_access%2Fverizon_subacct%2Fverizon_dsl#id=5&vid=f6d93c923b2821e88a7d1792f5f4341a&action=click

What you have is Homasote. Homasote is a brand name associated with the product generically known as cellulose-based fiber wall board, which is similar in composition to papier-mâché, made from recycled paper that is compressed under high temperature and pressure. You can make repairs to holes using drywall. Just make sure you get some the same thickness. Menards and Home Depot have cuttoff pieces for sale cheap.

https://www.hometalk.com/diy/repair/drywall/how-to-fix-a-hole-in-the-wall-44439068

https://www.hometalk.com/diy/repair/drywall/how-to-fix-a-small-hole-in-the-wall-3278948

https://www.hometalk.com/diy/repair/drywall/learn-how-to-patch-a-hole-in-drywall-22555804

https://www.hometalk.com/diy/repair/drywall/how-to-fix-a-small-hole-in-the-wall-diy-44440143

fill the hole with foam spray, then attach a small piece of drywall. Cut the hole bigger and squarer so that you will have a nice clean look when you spackle the edges and then paint

Hi Amber, hope this helps you out,

https://diy.stackexchange.com/questions/11993/how-can-i-patch-a-hole-in-a-painted-wood-paneling-or-plywood-wall

https://www.youtube.com/watch?v=Gwxfo7P5_24

Cut the hole into a square or rectangle, whichever works best. Add a piece of wood across inside then screw on your filler piece cut to the same size as the hole you cut out. Patch, sand, then paint.

Using some of the other ideas in this thread and some tips from this video might be the trick! https://www.youtube.com/watch?v=FCJz3JFT_HA