Forum OpenACS Q&A: Response to Can't load postgres.so - library not found. HELP!

This will probably solve your problem. Let us know if it doesn't. Taken from the documentation:

If you are running PG 7, make a symbolic link from libpq.so.2.0 pointing to libpq.so.2.1 because AOLserver looks for libpq.so.2.0 when
loading the driver:

cd /usr/local/pgsql/lib

ln -s libpq.so.2.1 libpq.so.2.0 (as user postgres)