There can be two reasons a dragon can't be placed in a habitat. First, the habitat already has as many dragons as it can hold. In this case, you'll either want to build a new habitat, or upgrade it if you can.
The second reason is that the habitat is not an appropriate color for your dragon. Dragons can only live in a habitat than share a color with it. For example, a Forest Dragon is green and therefore needs a Green Grove habitat. A Life Dragon is red and green, and could live in a Red Rocks or a Green Grove habitat.
You can check the colors of a habitat by tapping its Info button or by looking in the market. Look at the colored gem symbols under the habitat image. To check the colors of a dragon, you can tap the habitat it is in, and tap its button, or look in the market, and again look for the colored gems.