I see.
Anyway, as I was saying, there aren't any compatibility problems between 3.3 and 3.4.
You can import skeletons coming from Spine 3.3 or 3.4 into the latest Spine-Unity runtime and it will work just fine. Exports from 3.4 even work if you have a slightly older 3.3 runtime. So I don't think that's your problem.
The atlas format also hasn't changed in years so that's not a concern either.
If you're curious, you can find info on the version settings (how to set it) and more here: Spine editor and runtime version management
It should read "Could not automatically set the AtlasAsset for "{0}". You may set it manually."
That message just means it couldn't find the atlas automatically. This usually happens when the atlas is not the same name as the skeleton json or binary file, or is not in the same folder, or has the wrong file extension that Unity didn't recognize it.
But you can still set it manually. Just point it to where the atlas is. If you know for a fact that those regions are there, just set the AtlasAsset on the SkeletonData Asset.
Are you new to using Spine-Unity? Did you change certain export settings when you upgraded? (or notice they were changed?)
Does anything stand out here compared to your settings? (specifically for the Options section) Texture Packer Settings for Spine-Unity