according to chargaff’​s rule ,no.of purines=no.of pyramidines.then why uracil is not included[uracil is also a pyramidine]

Chargaff rule is basically applied only for DNA, not for RNA, because RNA normally does not exist in double stranded form. The chargaff rule is a consequence of complementary base paring of purines with pyrimidines, leading to double stranded neucleic acid. Since RNA is usually single stranded, it doesn’t apply there.