I get this error message when I go to places like:

http://{url}/node/807/map

As the source states:

/**
 * Implementaton of hook_install().
 */
function feedapi_mapper_install() {
  switch ($GLOBALS['db_type']) {
    case 'mysqli':
    case 'mysql':
      db_query("CREATE TABLE {feedapi_mapper} (
        `nid` INT( 10 ) UNSIGNED NOT NULL ,
        `mapping` LONGTEXT NOT NULL ,
        PRIMARY KEY ( `nid` )
        )");
      break;
    case 'pgsql':
      // Todo.
      break;
  }

It does not create the feedapi_mapper table in Postgres because it is pending for a to-do. (the 3rd line from the bottom.) Just so that you know, the following statement works in Postgres:

CREATE TABLE {feedapi_mapper} (
        nid int NOT NULL default '0',
        mapping text NOT NULL,
        PRIMARY KEY (nid)
)

My environment uses Postgres and the titled error goes away after I manually create the table as above.

My version is:
PostgreSQL 8.2.7 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)