Hi!
The lofts are surfaces at the beginning because you had to build each wall (internal and external) separately, in two steps (two loft features)... and that's ok, it's a correct approach.
Now, you have to turn these boundaries into a solid.
A good approach for this case is to use SCULP tool.
Just select the internal and external walls, and since they create a closed volume, sculp will turn this closed volume into solid.
Check the vídeo:
I don't have 27 installed, but could the first loft have been a solid, and the second loft been a cut of the first loft to make the inner opening instead of surfaces/sculpt?