I think the second argument of TriangleWave is supposed to be a variable, not a list. Maybe you want

In[4]:= TriangleWave[{-10, 10}, #] & /@ Subdivide[0, 1, 50]/2

Out[4]= {0, 2/5, 4/5, 6/5, 8/5, 2, 12/5, 14/5, 16/5, 18/5, 4, 22/5, \
24/5, 24/5, 22/5, 4, 18/5, 16/5, 14/5, 12/5, 2, 8/5, 6/5, 4/5, 2/5, \
0, -(2/5), -(4/5), -(6/5), -(8/5), -2, -(12/5), -(14/5), -(16/5), -(
  18/5), -4, -(22/5), -(24/5), -(24/5), -(22/5), -4, -(18/5), -(16/
  5), -(14/5), -(12/5), -2, -(8/5), -(6/5), -(4/5), -(2/5), 0}