Interestingly, it looks like my issue with the spawner relates to secondary levels. My initial level works fine but nothing on my second level.
If you find it isn’t working on your initial level then something I read about was going into edit on the spawner, double clicking on your foliage type actor, then checking the ‘Height’ Min and Max to ensure it’s within appropriate range. In my case my foliage seems to be min -262144.0 and max 262144.0 which certainly more than covers the range I need.
I found that could get rid of the “Unable to spawn instances. Ensure a large enough surface exists within the volume.” message by making the spawn area twice the size that I needed it, though nothing got generated on the second level.
Are you working on a single level or multiple as part of a tiled terrain?