PLS-00201: identifier … must be declared

connect / as sysdba

drop   user usr_1 cascade;
drop   user usr_2 cascade;
drop   role rol_x;

create user usr_1 identified by pw;
create user usr_2 identified by pw;

grant
   create session,
   create role,
   create procedure
to
   usr_1;

grant
   create session,
   create procedure
to
   usr_2;

connect usr_1/pw

create function func return number
   authid definer
is begin
   return 42;
end func;
/

create role rol_x;

grant execute on func to rol_x;
grant  rol_x to usr_2;

connect usr_2/pw

begin
   dbms_output.put_line('func = ' || usr_1.func); 
end;
/

create procedure proc
   authid definer
is begin
   dbms_output.put_line('func = ' || usr_1.func); 
end proc;
/

show errors
--
-- Errors for PROCEDURE PROC:
-- 
-- LINE/COL ERROR
-- -------- -----------------------------------------------------------------
-- 4/4      PL/SQL: Statement ignored
-- 4/38     PLS-00201: identifier 'USR_1.FUNC' must be declared


connect usr_1/pw

grant execute on func to usr_2;

connect usr_2/pw

begin
   proc;
end;
/