amazon web services - APEX_ADMINISTRATOR_ROLE in AWS RDS Oracle Instance -

i trying install apex on aws oracle 12 rds instance. in order achieve this, following these instructions : http://docs.aws.amazon.com/amazonrds/latest/userguide/appendix.oracle.options.apex.html

however, got stucked in step 7:

step 7:

you must set password apex admin user. this, use sql*plus connect db instance master user, , issue following commands:

grant apex_administrator_role master;

@/home/apexuser/apex/apxchpwd.sql

replace master master user name. when apxchpwd.sql script prompts you, type new admin password

when log my rds instance master user , execute this:

grant apex_administrator_role [mymasteruser]; 

i received error:

error @ line 1: ora-01924: role 'apex_administrator_role' not granted or not exist 

can please me solve this?

edit 12/09/2017.

using post/answer:

https://serverfault.com/questions/276541/how-do-you-recover-you-rds-master-user-username 

i understand master user shown in following image. know, in rds instance have no access sys or system user, user can use.

many thanks

edit 20/09/2017.

i applied alex solution, , works!!. however, issues comment:

the tutorial changed, in fact url changed, is

http://docs.aws.amazon.com/amazonrds/latest/userguide/appendix.oracle.options.apex.html (the last "html" in uppercase before)

but not reliable now, there points should fixed, e.g. says rds support oracle apex version 5.1.2, tried versión , got error:

also, directories dont match previos step ....

so, used versión tutorial says : oracle apex version 4.2.6.v1

i had execute both statements :

exec rdsadmin.rdsadmin_util.grant_apex_admin_role;  grant apex_administrator_role [master]; 

then execute apxchpwd.sql script successfully!!.

but, unfortunately, when accessed apex home page , tried create new workspace "ws_prueba", receive error (im trying create apex admin user):

any ideas?

use

exec rdsadmin.rdsadmin_util.grant_apex_admin_role; 

instead. have case open on aws , asked them update documentation page.