I worked with a guy in the Navy who was illiterate.
He wasn’t stupid, he excelled at his job .
Could not read to save his life though and eventually stagnated at E-5 ,he couldn’t pass the written exam .
He probably would have aced an oral exam.
I knew an EM like that, couldn’t pass the E-6 exam. Eventually he received a command advance to E-6.
I knew a Sailor who made it to E-6 but scored so low on every exam he took that he was never selected for Chief. He knew his rate better than most. If I recall, he maxed out on award points, had many achievement awards, and had two Warfare pins. He barely made it to E-6. He did because of the points from the awards and pins. He would have been a senior SNCO in the Corps or Army.
That sounds like an un-diagnosed case of dyslexia.
We had a staff sergeant in our squadron who was very sharp and fortunately his dyslexia was correctly diagnosed. It caused issues but his awareness of it helped him mitigate it's difficulties.
He was able to select a viable career track, at least to tech sergeant, when he PCS'd from our base.