How much do you weigh? If you're heavy then losing weight will help as body exercises are harder when you weigh more.
If you can't do a plank you probably have a weak everything, not just a weak core. You'd be better off training your whole body rather than just core. But to answer your question directly...
You can start with a regressed version of the exercise which makes it easier. You can use a piked body shape (sticking your bum in the air) and gradually work towards a flat body shape as you get better at the exercise. The only thing you don't want to do is plank with an arched back as that can cause pain over time in some people.