Hey there global user,
We understand you're trying to fill shapes with colors within the Notes app. We'll be happy to help.
In order for the plus sign to appear so you can enter Markup, you'll need to have an image inserted into the note you're editing. You can do that by tapping the camera icon in the Notes app tool bar.
Once you have have an image added, tap on the image, then the pen icon to enter Markup. From there, you'll see the plus sign and you can select a shape. Once you add the shape, you can adjust color and if you want to fill it solid.
For full details, check out the section "Draw a shape" here: Draw in apps with Markup on iPad
Cheers!