Your system is not doing any work. You can add a body force:
fbody = Expression(('sin(x[1])', 'cos(x[0])', 'x[0]*x[1]'), degree=2)
F_body = inner(fbody, v)*dx
# Definir la ecuación de equilibrio variacional
F = F_elastic + F_pressure - F_body
On the other hand, since you are new to FEniCS, I recommend you to use the new DOLFINx, which has a really nice tutorial (or this) and a very rich documentation. There are some results here about hertzian contact in DOLFINx, and here for the Legacy FEniCS.
Hope this helps.