Making a heart bokeh with a Nikon D5000?

I have the heart construction paper over the lens, i'm using a 200mm lens, the shutter speed is 1/20 sec, the aperture is f5.3(I can't make it any lower and I don't know why, I know it's supposed to be like f1.8) but I don't know how to make it lower? I think this might be the problem because it looks great when i'm about to take the picture, it focuses on my subject but the camera doesn't let me take the picture because it says "subject is too dark" since the construction paper is black. When I remove the construction paper, it takes the picture but I want the hearts.

Maybe you're too close to the subject, to the point the camera doesn't focus and doesn't take the shot.
It should matter to the camera if the subject is too dark or not.

Your zoom lens will never stop down to 1.8.Is the camera on a tripod? Are you using manual focus? I suggest you spend some time at your local library reading some of the basic photography books…

Try increasing the ISO until it can get a good exposure. Although if you are in manual mode it should just shoot.

You need a bigger aperture. Try this website, it seems to explain the process really well: http://www.diyphotography.net/diy_create_your_own_bokeh

You're zoomed in too much, you need the aperture wider, that'll only happen at the wide end of you're zoom.

Next, you're going to have to read the manual. You need to take control. You'll need to do these things at the very least: Centre Focus and Spot Meter. Ideally you want to be in M (manual) mode so you control ISO, shutter and aperture.

Don't worry too much about ISO, it'll be noisy, but it won't matter in the image you're taking. Set it to 1600. You'll need the sensitivity. Then try a shutter speed of 160, zoom to 50mm and get as close to f4 (lower would be better, but that's for high end lenses) as you can. Now try to take the picture. Not good huh? Now you have to alter those parameters, they are all interdependent, so each has a relationship with the others. Try a shutter speed to 80 this time, 40 the next.you'll start to see how the longer exposure is letting more light in, but losing you a degree of crispness.

Also, How small is the heart you've cut? It can be surprisingly large and the effect will still work, or you could use multiple hearts to get more light to the lens.