"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? :)

"Capital" is uncountable, so your sentence is correct. You would use the plural form for countable nouns. For example, they don't have enough chairs.