Possible Solution:

• Mathematica Guru(Jedi) will come here and solve your attempt.
• Put your question here maybe they will answer your questions there.
• You must upgrade to version 11.3
• Use other 3d software.Blender 3D is free and can export to stl files.
• or something else..

Regards,MI

Maybe is a limtation of file format or something else.

With dxf file format works fine:

  t = Graphics3D[{Cylinder[{{0.7, 0, 1.4}, {0.1, 0, 2.1}}, .1], 
  Ball[{0.1, 0, 2.1}, .1], 
  Cylinder[{{0.1, 0, 2.1}, {-0.4, 0.6, 1.6}}, .1]}]

  Export["test.dxf", t]
  Import["test.dxf"]

Another workaround:

   a = Cylinder[{{0.7, 0, 1.4}, {0.1, 0, 2.1}}, .1];
   b = Ball[{0.1, 0, 2.1}, .1];
   c = Cylinder[{{0.1, 0, 2.1}, {-0.4, 0.6, 1.6}}, .1];

   a1 = BoundaryDiscretizeGraphics[a, 
      MaxCellMeasure -> {"Length" -> 0.05}];(* Discretize with a maximum edge length of 0.05*)
   b1 = BoundaryDiscretizeGraphics[b, 
      MaxCellMeasure -> {"Length" -> 0.05}];
   c1 = BoundaryDiscretizeGraphics[c, 
      MaxCellMeasure -> {"Length" -> 0.05}];

    reg = RegionUnion[a1, b1, c1]
    Export["test.stl", reg]
    Import["test.stl"]

Probably,the error occurs because the figures 3D penetrate each other.Yes the same error on my MMA 11.3 version.

Workaround is using RegionUnion function:

c = Cylinder[{{0, 0, 0}, {0, 0, 1}}, 1]; b = Ball[{0, 0, 1}, 1];
t = RegionUnion[c, b];
Export["f:\3.stl", t]

Visualize it:

 RegionUnion[c, b]//Region

Execute code:

 b = Ball[{0, 0, 1}, 1];
 p = Prism[{{1, 0, 1}, {0, 0, 0}, {2, 0, 0}, {1, 2, 1}, {0, 2, 0}, {2, 2, 0}}];
 t = Graphics3D[{b, p}]
 Export["f:\\3.stl", t]

And what Yours conclusions ?