Hi,
I would suggest you look through the Q&As, where you should be able to find that this issue has been raised before. This is due to a recent update of pdb format by the Protein Data Bank (as I remember, at the end of last year?). In the new format, bases DA, DT, DC, and DG are specified for DNA, while A, T, C, and G for RNA.
What you can do is:
1. Go the the 3DNA folder at where you installed.
2. Find the subfolder BASEPARS, and get in.
3. Open the file baselist.dat. In the file you will see basically two columns of residue names.
4. At the bottom of the file, add
DA A
DT T
DC C
DG G
5. Save the file
Retry find_pair, and you should be fine.
Hope it works!
Guohui Zheng