Complex sentence: Since, N-hydroxysuccinimide reacts readily only with aliphatic amine groups, which nucleic acids lack, nucleotides have to be modified with aminoallyl groups.
Split into simpler sentences: Since, N-hydroxysuccinimide reacts readily only with aliphatic amine groups that nucleic acids do not have. For that reason DNA or RNA nucleotides have to be modified with aminoallyl groups.