How do I fill a key?

Thank you for the screenshots. I'm assuming that your PNG image, Layer 1, does not have transparency. If that is the case and you have a nice solid color Linear Key is a pretty ham-handed way of pulling a key, but it will work. I took a screenshot of your screenshot and I'm using that as layer 1. Here's how that looks with one click on the background color:

Here's one click on the Foreground color and it takes more samples so I wouldn't do that.

Notice the transparency grid is on so I can clearly see through the layer. Any layer I put below Layer 1 will now fill the hole. To invert the matte all you need to do is add Invert to the layer and select the alpha channel.

Personally, if I needed to pull a key from this image I probably would have used Curves + Keylight then Invert/Alpha if I needed a hole where the logo is.

You need to do a little image manipulation to get keylight to work but you have a lot more control over the edges.

If it was just a still image I would open it in Photoshop and remove the background and be done with it. It would save render time.

Solo your layer, check the transparency grid, and mouse over the transparent areas with the Info Panel open to check for the accuracy of the key. It should be 1 if your Info Panel is set to read decimal or 255 if it is set to 8-bit.

We need screenshots showing the modified properties of the layer that is giving you problems. Select the layer, press the 'u' key twice, take a screenshot of the entire UI and embed it using the toolbar so we can see what is going on.

In the meantime, please check out the User Guide. There are tutorials in there that show you how to use keying to create transparency.

It is all about layers. Please spend some time with the User Guide. Don't just jump into YouTube looking for tutorials. You should also open up the Learn workspace and spend at least the first 6 minutes running through the workspace. AE is not easy to use or intuitive. You have to get some good training.

Think of After Effect like you are creating a collage with a bunch of photos from a magazine and a pair of scissors. You cut out the images using keys or mattes or masks, then you stack them on top of each other. You don't edit in After Effects, you create shots that can be rendered and edited in an NLE like Premiere Pro.

Keying effects make parts of the layer transparent, so any layer below the keyed one will be visible through those transparent parts. So: put any layer below the keyed layer to "fill" it.