You will recall that in the usage of articles in English, the articles “a” and “an” generally can be used only with count nouns, as in “She wants a piece of bread” and “He asks for an hour of silence.” On the other hand, the article “the” generally can be used with noncount nouns or can be omitted entirely, as in “They had an argument over the land” (referring to a specific piece of land) and “They flew over land” (referring to land as a surface).
As to the noun “lack,” it just happens to be a noncount noun that can use either the definite article “the” or the indefinite article “a”—and in some cases no article at all—for particular usages of that noun. In fact, whether to use “a” or no article at all is oftentimes a matter of personal preference of the author. In my case, as a matter of style, I prefer not using “a” in the particular sentence you presented: “There is still lack of awareness of the social problems encountered by the physical disabled.” My basis for that choice is that both the word “lack” and “awareness” are noncount nouns, and that the sentence reads perfectly in order even without the article “a.” This doesn’t mean though that its use in this sentence that you presented is grammatically wrong: “There is still a lack of awareness of the social problems encountered by the physical disabled.” It’s just that I feel “a” is uncomfortably extraneous in that construction. (On the other hand, I wouldn’t hesitate to use the article “a” for this sentence using the noncount term “lack of evidence”: “A lack of evidence has jeopardized the prosecution of the suspected thief.” It’s because the noun “evidence” is clearly countable in this particular case.)
As to the other sentence you presented, the article “a” is obviously needed between “give” and “20%”: “If passed, House Bill 12147 will require all transportation services, hotels and lodging establishments, theaters, government hospitals, and other medical institutions all over the country to give a 20% discount on PWDs’ purchases.” Anyway we look at it, the term “20% discount” is undoubtedly a count noun, so not to use the article “a” for it definitely will make the sentence look and sound out of kilter.
