What does this error mean, please?

[SEVERE] Could not set generator for default world 'world137': Plugin 'TerrainControl v2.4.14' lacks a default world generator

Marked this as "Won't fix", since this cannot be fixed by us. CraftBukkit displays that message when Terrain Control says it doesn't want to generate that world, which happens on TerrainMode:Default.

What is the value of the TerrainMode setting in the WorldConfig?

"Default"

Hmm, did a little digging and it seems that Bukkit/CraftBukkit@8205884 is the cause of this issue. It shouldn't hurt anything, as Terrain Control will still work.

You might want to use TerrainMode:Normal, as the default settings look exactly like the default generator.