You are trying to import a binary file, not the actual dbinit file.
For Postgres, the file you need to import is named dbinit_pgsql.sql.
The documentation (https://www.netxms.org/documentation/adminguide/installation.html) gives the following sample command (your actual paths will likely differ):
/usr/local/bin/nxdbmgr init /usr/local/share/netxms/sql/dbinit_pgsql.sql
