It seems like you have a complex geometry which can’t be meshed easily. Please try to simplify the geometry as much as possible, otherwise you may run in to one of these problems while meshing. You can also try to merge the solids in to one which can be altogether a separate part with a single material assigned to it. For this you can have a look at this forum post here.
Once done try to mesh it again and I hope than it will work. If you have any question/s, feel free to ask.