We all know at least one language. Their own. English is mastered by the English and Americans with different mindset, both smart and foolish.So the matter, of course, is not in abilities.
This requires time and effort. And if a person does not need this for work, nor does he intend to go anywhere, then he does not see any meaning.
But if he goes to other countries, you need to read some documentation, instructions, and he doesn’t want to teach anything, then his excuses – I don’t have the abilities – look really unfortunate.
I remember as in high school, explaining my assessments in English.
A few years later, after finishing school, he safely mastered it.
Many people were ruined at school, by faultless teaching, when grammar was blurred for several years.