Why can't I select current SCN?
Users by default do not have the privilege to use those objects:

[oracle@o7ca1 ~]$ sqlplus / as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Mon Mar 4 20:42:48 2019
Version 19.2.0.0.0

Copyright (c) 1982, 2018, Oracle.  All rights reserved.


Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.2.0.0.0

SQL&gt; show user
USER is "SYS"
SQL&gt; select current_scn from v$database;

CURRENT_SCN
-----------
     564558

SQL&gt; create user u1 identified by u1;

User created.

SQL&gt; grant create session to u1;

Grant succeeded.

SQL&gt; conn u1/u1
Connected.
SQL&gt; select current_scn from v$database;
select current_scn from v$database
                        *
ERROR at line 1:
ORA-00942: table or view does not exist


SQL&gt; select dbms_flashback.get_system_change_number from dual;
select dbms_flashback.get_system_change_number from dual
       *
ERROR at line 1:
ORA-00904: "DBMS_FLASHBACK"."GET_SYSTEM_CHANGE_NUMBER": invalid identifier


DBMS_FLASHBACK


  DBMS_FLASHBACK Security Model
  
  To use the DBMS_FLASHBACK package, you must have the EXECUTE privilege
  on it.


About Dynamic Performance Views


  After installation, only user SYS or anyone with SYSDBA privilege has
  access to the dynamic performance tables.


We do not know what privileges you had before, that you do not have now.

Ask your DBA to grant you the required privileges.

grant read on sys.v_$database to petar;


Or:

grant execute on dbms_flashback to petar;