what do you do when you cannot afford a new bed

If it’s one that can be flipped, try flipping it. If not, move it so the foot and head are switched. You can also get a mattress topper. Their prices start around $20.
There’s a second hand shop here that gives people all kinds of furniture if they can’t afford it, so you could look around in your area for something like that

If it’s on a box spring support, try putting plywood under the mattress.

Look on Facebook local area, deceased estates,

I don't have one

Ask around to people you know if they have a mattress they can give you

A good memory foam overlay might help bridge the gap until you can save enough$

What you can do is place some old pillows under the mattress in the center which will elevate the center section and make the top surface level as well as supportive all the way across.

Can you set aside a fund for it? Every week put 10-20$ in a jar?

Flip and rotate the mattress every 6 months.

don't your parents have Costco memberships?