Evidently this is binding to an old version of libpq.so that lacks
that symbol. Probably it's finding some vendor-provided libpq.so
in /usr/lib or /usr/lib64, whereas what you need is to find the
one in /usr/lib/postgresql/11/lib. Applying "ldd" to the psql
executable would help confirm that diagnosis.
It's hard to say more than that without knowing anything about
how this installation was built.