Just reproduced your problem and got the same message. What you need to do is ungroup the text object, then weld the characters into a single object, then perform the ngon operation as above. I know that Hexagon can be extremely temperamental. It can do what you want it to, it just needs a little patience.
1: Ungroup as such.
2: Weld new characters and perform ngon slice as above. Your new mesh will be subdivided like so.
3: Import to Terragen and enjoy happy time.