"Capital" is an uncountable noun (in the sense of money) so it doesn't have a plural form ("capitals" doesn't exist).
"Enough capital" is correct.
This works for all uncountable nouns.
Eg. We didn't have enough sand to build a castle.
Do you have enough flour to make pancakes?
But remember, for countable nouns you must add "s"
eg. We didn't have enough brickS to build a castle.
Do you have enough eggS to make pancakes?