Can Someone help indicate what I have done wrong with this model??

It is a simple model which is a shell  / cover and I want to fix the base end of it and apply a load to exterior surface. Should really be pretty simple. But every time I attempt to run the simulation it comes up with an error saying that 'no restraints defined' and when I select the base face of the model (the only flat face) at first it accepts the selection however once you attempt to run the simulation an error comes up within the 'Fixtures' selection and it will not allow you to select the face that needs to be fixed and comes up with a 'select entity on shell' error.

It meshes well and I have done a couple of similar models that do run the simulation like this so I am unsure why this one will not??

Please help.