You are removing all internal attributes by assigning (some) child modules to a list:
self.vgg16 =list(self.vgg16 .children())[0][:-1]

and thus cannot use the .features attribute anymore.
If you want to remove the last linear layer, you could replace it with an nn.Identity module.
Also note, that you should use an nn.ModuleList instead of a list so that all parameters will be registered.