A solid cannot be created.

[maxim.gutter]

Hello. I have a 3D-inclined polyline. How can I create a solid body from this line? I always end up with a surface.

[tramber]

Your object is not planar 😉

(my opinion, i saw only your Z values at each point)

Would it be, it could not become what we really call a solid body, only a planar one or a region.

Do you need some help to make it so ?

You can also treat by pairs of triangles.... a first step to where you seem to go....

[parkr4st]

attached with 3 objects 2 solids. one surface

one solid is 3dpolylines lofted along a 3dpoly central path

one is the right side a 3dpoly SWEEP on central most cross 3dpoly

THICKEN surface to 0.1 units

One is missing a small area at the lower end - may need a small solid unioned to it

suggest you experiment to get the wanted result 

I like the curved surface best although it may not fit your needs