Is there some significant portion of the population who lack abstract or procedural thinking skills?

Pretty well everyone under the age of 12, so call it 15% of the population. Quite a few over that age, but that’s not something that I can tell you. Nor can I infer what you consider to be significant.

Young children have almost exclusively “concrete” reasoning, not abstract. But it’s not as simple as it sounds, they seem to be able to process the aim, feelings and intentions of imaginary friends, teddy bears, Santa or cartoons, so it’s not a simple “no”.

HOWEVER, this may not be what you are asking, I may have misinterpreted your aim or your terminology, or educational psychology may have moved on since I studied in 1993.