Calling schema specific packe/function from PL/SQL

Question

From PL/SQL I am trying to call a package function in a specific schema (see below). When I try it gives me this error:
Invalid function body condition: ORA-06550: line 3, column 15: PLS-00201: identifier 'SCHEMA1.SIS_EXPRESS' must be declared ORA-06550: line 3, column 1: PL/SQL: Statement ignored
Am I missing something as I thought this was the correct syntax?
DECLARE button_ok boolean := FALSE;
BEGIN
button_ok := my_schema.SIS_express.ok_to_add (:APP_USER,:P0_LOGIN_SCHOOL_YEAR,:P0_LOGIN_SCHOOL,:APP_ID,:APP_PAGE_ID);
<<end_of_trigger>>
return button_ok;
END;

Answer 1

An Apex workspace is associated with one or more schemas.
An Apex application in that workspace needs to select one of these schemas as its parsing schema.
This means that all application code (PL/SQL and SQL code you write in that Apex app) will be parsed (using the DBMS_SYS_SQL package) as the parsing schema.
Note that the current and acual schema for the Apex database session will the anonymous schema account as configured for connectivity for that database access descriptor (DAD) on the Apache server. You do not connect (via Apex) as your parsing schema. You connect to an anonymous schema. It runs trusted Apex code (that executes with the Apex schema's privs). This code parses your Apex app code via the system package to be executed as your parsing schema.
So you need to make sure that your parsing schema has the appropriate rights to execute the PL/SQL and SQL application code you put into your Apex application.
Also be aware that if you use that DAD to directly execute your PL/SQL code in the database (procedures or functions), the call will be made from the anonymous database schema used by that Apache DAD connection - thus that schema will then need to have exec rights on your code.
In the case of the Apex flow engine - public exec rights exist on it and thus that DAD session from Apache can logon as the anonymous schema and execute the Apex run-time to run an Apex application.