The Mroonga storage engine includes a number of user-defined functions that need to be created before they can be used. If these are not created already during Mroonga setup, you will need to do so yourself. The full list of available functions and the statements to create them are found in share/mroonga/install.sql, for example, as of Mroonga 7.07 (MariaDB 10.2.11 and MariaDB 10.1.29) running on Linux:

DROP FUNCTION IF EXISTS last_insert_grn_id;
CREATE FUNCTION last_insert_grn_id RETURNS INTEGER
  SONAME 'ha_mroonga.so';

DROP FUNCTION IF EXISTS mroonga_snippet;
CREATE FUNCTION mroonga_snippet RETURNS STRING
  SONAME 'ha_mroonga.so';

DROP FUNCTION IF EXISTS mroonga_command;
CREATE FUNCTION mroonga_command RETURNS STRING
  SONAME 'ha_mroonga.so';

DROP FUNCTION IF EXISTS mroonga_escape;
CREATE FUNCTION mroonga_escape RETURNS STRING
  SONAME 'ha_mroonga.so';

DROP FUNCTION IF EXISTS mroonga_snippet_html;
CREATE FUNCTION mroonga_snippet_html RETURNS STRING
  SONAME 'ha_mroonga.so';

DROP FUNCTION IF EXISTS mroonga_normalize;
CREATE FUNCTION mroonga_normalize RETURNS STRING
  SONAME 'ha_mroonga.so';

DROP FUNCTION IF EXISTS mroonga_highlight_html;
CREATE FUNCTION mroonga_highlight_html RETURNS STRING
  SONAME 'ha_mroonga.so';

DROP FUNCTION IF EXISTS mroonga_query_expand;
CREATE FUNCTION mroonga_query_expand RETURNS STRING
  SONAME 'ha_mroonga.so';