Can you squat with a light weight? Do you deadlift? As Benny asked - what is wrong with your back?
Obviously I don't disbelieve you personally, but it's hard to give advice with limited info. I have had A LOT of people complain to me about a bad back and they don't REALLY have anything that is not correctable. They just have either poor movement patterns or they are just plain weak and don't know it. The advice I give these people is to do what they believe they can't, and step out of their comfort zones and stop lying to themselves. But this doesn't mean to be STUPID. It means - correct the problem. Start with a light weight and learn everything you can about form and practice, practice, practice that form.
On the other hand - I've also had people with steel in their backs, damaged spines, legit hip or knee issues, etc. - and obviously the advice here will be a little different.
If you are having pain and can't train effectively, steroids will make you stronger, which will amplify whatever you are doing to cause pain in your movement.
One of the best pieces of advice I have for people is to let go of their ego, drop as much weight as needed off the bar and just rep out for form. I think 95-135 pounds is about right for most people here. Just do even a 5 X 5 for squats with 95-135 every other day, for a few weeks. Go to Youtube and lean all you can about the movement - AthleanX is a reliable source of info - and then after 3 weeks or so, just slowly start adding weight on a regular basis. You will get stronger without struggling a lot of the time and your improved form will mean you have less pain doing it.
And if you cannot do that for a more serious reason - then no problems. Just because everyone says "You have to do X exercise" doesn't mean you do. You are TRAINING to be a better version of YOURSELF. You don't have to do any exercise that isn't right or working for you. As long as you're not avoiding an exercise just because it is hard or you have to learn (in other words - be true to yourself) - don't worry about it. Leg extensions, leg presses, leg curls, lunges, hack squats, whatever - there are a ton of exercises out there. Find what you can do without pain and work it.