"incapable of carrying a tune"; "he is incapable of understanding the matter"; "incapable of doing the work"
is similar to
(usually followed by `to') lacking necessary physical or mental ability
"dyslexics are unable to learn to read adequately"; "the sun was unable to melt enough snow"
see also
not qualified or suited for a purpose
"an incompetent secret service"; "the filming was hopeless incompetent"
is an attribute of
the quality of being capable -- physically or intellectually or legally
"he worked to the limits of his capability"
is an attribute of
an aptitude that may be developed