If you are incapable of something, you cannot do it at all — the ability is missing, not just the mood. A newborn is incapable of walking; the old printer is incapable of colour.
The grammar is fixed: incapable is followed by OF plus a noun or an -ing form — incapable of lying, incapable of holding a conversation. 'Incapable to do it' is wrong; the word you want there is unable. Standing alone, calling a person incapable is an insult about competence: 'the man is completely incapable.' Doctors and lawyers use it for someone unfit to manage their own affairs, and their noun is incapacity; the plain noun is incapability. The opposite is capable, and it takes OF in exactly the same way.
1adjectiveincapableNot able to do something, because the ability is not there.
“The engine is incapable of pulling that weight uphill.”
2adjectiveunableUnable to do things properly; useless.
“He treats every new assistant as though she were incapable.”
3adjectivefitNot fit to look after your own affairs.
“The court found him incapable of managing his own finances.”
Incapable came from Middle French incapable and comes apart cleanly: in-, 'not', on the front of capable. Behind capable stands the Proto-Indo-European root keh₂p-, the seizing-and-holding root that Latin capiō grew from. What you are incapable of, you cannot get hold of.
Proto-Indo-European
*keh₂p-
Middle French
incapable
English, today
incapable
Middle French incapable, equivalent to in- + capable.
Etymology from Wiktionary, CC BY-SA