Depends on what kind of layers.
If the layer is not selected, select the layer you want to draw on.
If that's the case it should work. If not...
Try merging them, or rasterize them if they aren't already.
When you choose some of drawing tools like Pen Tool, Shape tool, take a look at Options Bar below menu. At Options Bar you can choose to draw on new layer (blank except what you will draw) and this is recommended because content can be changed at any time, to draw only path or to draw pixels on existing layer. Those options are on the very beginning of Options Bar, navigate mouse and hold under options to show you pop-up tooltip.
When working with Brush Tool, Pencil Tool or Mixer Brush Tool you dont have this options because Brushes can paint only pixels but you have option to make new layer and paint on it so if you want to change some quality of painting or delete that layer you can do that at any time even after saving image (save in psd).
Yes, .psd files maintain the layers you create in Photoshop so that the next time you open your .psd file in Photoshop you can continue to work directly onto the layers.
layers are separate images stacked one above another, layers can have transparent areas so you can see content below
Layers are pictures stacked one to another, Layers can contain transparent pixels so you can see through Layer above what is on Layer below.
Layers are separate images which can contain transparency and can be combined as one single photo. Photoshop offers many kind of layers: blank, type, shape, fill and so on. Layering means you have multiple layers in document and you can manipulate with them: with their position, transparency, also you can group, ungroup them for better organized document and so on.
A drawing tool is like a pencil or paint brush etc. So that it helps you draw.
Basically Layers are pictures stacked one above another, Layers supports transparency so you can see what is below. See related link below answer.
To Flatten your Layers in Photoshop simply go Layer>Flatten. I recommend saving your image as a psd before you flatten so that you can refer back to your layers. Then flatten and save as a jpg.
If the Layers palette is missing, you can return it by clicking on the "Window" menu near the top of Photoshop, then selecting "Layers" to ad a checkmark next to it, indicating that it is now visible. Photoshop also allows palettes to be un-docked from the program and dragged around anywhere on your screen, not just in the Photoshop window, so if you have a dual-monitor setup, make sure that you didn't somehow hide the palette on your second monitor.
For me i use it to scan what i drew to digital draw it on my computer , with such as photoshop , so i get neat results.
Photoshop is an amazing program. I have been using it for many years and so far the only limit, as far as I am concerned, to the program is your imagination. I just finished a graphic project which contained 127 Layers. There really is no limit to the amount of layers you can use. You will need a decent computer to take full advantage of the program.
There are a lot of software which can help you. The most famous are Photoshop, Corel Draw and so on.
The first layer is your background. When you add a new layer, it's like a clear sheet of plastic. You can draw on this plastic and not edit the layer below it. If you add a third, it doesn't change the two below it. So you can tweak each layer, and if you mess up, you haven't destroyed your entire project.