I’m adding this in case anyone is searching for help with this:
It’s not obvious that you have to highlight the foliage thumbnails and RMB > Activate before the brush will show up in the viewport when using Paint foliage on landscape.
This may not be a problem if you are starting with a new Foliage, but you may come across this if you load a scene that already has instanced foliage painted and you want to continue adding more.
From reddit: u/aeroson
My issue was that the foliage brushes were deactivated
So I had to select and activate them
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