I believe that should be username and password per above link:
"CREATE USER MAPPING FOR postgres SERVER odbc_server OPTIONS (username 'root', password ''); "
Yes.
Does 'yes' mean you changed this:
CREATE USER MAPPING FOR pg SERVER odbc_db2 OPTIONS (odbc_UID 'db2inst1', odbc_PWD 'db2inst1');
to this?:
CREATE USER MAPPING FOR pg SERVER odbc_db2 OPTIONS (username 'db2inst1', password 'db2inst1');
If i try to use username it throws error as invalid option
postgres=# CREATE SERVER driver_db5 FOREIGN DATA WRAPPER odbc_fdw OPTIONS (dsn 'sample'); CREATE SERVER
postgres=# CREATE USER MAPPING FOR pg SERVER driver_db5 OPTIONS (username 'db2inst1', password 'db2inst1'); ERROR: invalid option "username" HINT: Valid options in this context are: <none>