I can’t do a crunch worth my life. What can I do so my body will be able to do one?

Crunches barely train your abs and can even be harmful to your lower back. Better core training would be planks, mountain climbers, and lifting weights with proper technique and breathing will work tremendously.
Remember strength is gained in 5 reps, so doing 20 reps of a thing will not make you stronger but it will develop muscles mass, and cardio. Read some pavel tsatsouline or 5×5 method for more on building strengths – easy reads and easy to implement in your routine.

Recovering from a surgery takes time. I need time and discipline to get back to the level I was before and from there start to plan to get even better than ever.

Small approximations! Have a friend hold your feet down and crunch up as far as you can. Rest 30 seconds and do it again. Tomorrow, do it 4 times!

Start small. Do half crunches and hold them for 5 seconds, instead or rushing to do the next half crunch. Slow, sustained movement will build muscle better than fast, rushed movement.

It would depend on why you can't do a crunch. If it's a flexibility issue, find exercises that will increase that flexibility. If it's an issue of your core muscles not being strong enough, I would recommend other exercises that work those muscles. There are plenty of workouts that strengthen the abdominals and core that you could do as an alternative. I've found youtube great for this because you can see it demonstrated.

Start with yoga! I lost the ability to do crunches for a while about a year ago I started doing easy yoga for about 15-20 minutes every day and in about a week I was able to do crunches again.

Just start trying doing one, practicing regularly you will see improvements in your muscles strenght. Start small, but make the effort to reach for more

Start small, just get your shoulders off. And little by little it will come. I don't think I ever do my workouts well, but I do what I can and I'm getting a bit better week after week!
Also, try Pilates. The Pilates abs workout are super different and soft and great to start if you really have no abs at all.
Also, suck in your stomach during the day, as often as you can and as long as you can. I started doing that when I was 10 and now sucking my stomach in is my default position. It's good for the posture as well 🙂

Try doing an inverted crunch. Rather than trying to bring your chest up to your knees keep your back flat on the floor with your arms laid by your side, palm down, and bring your knees slowly yo your chest and back to start position.

Well don't feel to bad because I can't even do a push up! But I've learned that just doing little things will help me achieve my goal. Sometimes when I'm lying around watching television I just get up and start doing jumping jacks. I don't start doing it everytime just when I think about it. Little things like this help me stretch and build more muscle. And everyday I try a push up and get closer and closer to doing it. So try the little things and try crunches everyday! But dont forget the diet and nutrition is also a big part in a healthy body!

Neither can I, not a proper one anyways. But I try to do them as part of my morning excercise, and while I couldn't even lift myself whatsoever a little over a week ago, I'm now just about halfway up from training those muscles the little bit that I was able to every day. Keep trying, because you'll inevitably start to see progress, and when you do you'll be that much more driven to see it through. The feeling is indescribable, and you deserve to have it.

If a sit up or crunch is difficult for you I would start even smaller first. Walking helps to engage your core muscles that you use when you do crunches. So start going on walks and see how that feels and then make your way to doing crunches. You and your body are stronger than you think so keep at it! You totally got this!

Start with simply stretching! Google stretches for your abdomen hips and lower back. Build strength and flexibility through regular stretching. Maybe even consider yoga!