How do export a weapon model from blender to the tes construction set?

You need http://niftools.sourceforge.net/wiki/Blender. With that, after makin a mdoel, go select File -> Export -> Netimmerse/Gamebryo. There you can select Morrowind and output folder.

I kept getting this error. Error:texture T must be of type IMAGE or ENVMAP
Is there something I forgot to set?

Did you properly add texture to the model? I always do it this way:

Materials (F5) Link to Object -> Add New, Texture -> Add New, Map Input -> UV
Texture (F6) Texture Type -> Image, Image -> Load