Insight and CHARMM used different names for the nucleotides; the error message indicates that there is a residue named "A" in the Insight pdb file, but CHARMM expects the name ADE, as is also clear from gen-dna.inp.
If you used gen-dna.inp exactly as posted there are also other inconsistencies in your atcg.pdb file. The segids should be A and B (not DNA1) and there should be no 5' or 3' phosphate groups in the structure
! No end phosphate groups
gene a first 5ter last 3ter

There is a tar-file containing my examples, and the pdb files they use, at:

Lennart Nilsson
Karolinska Institutet
Stockholm, Sweden