Begin
....
...
...
...
...
if condition then
fnd_message.set_string('Message');
fnd_message.show();
else
null;
end if;
end;

SQL> Begin
  2  if 1=1 then
  3  fnd_message.set_string('Message');
  4  fnd_message.show();
  5  else
  6  null;
  7  end if;
  8  end;
  9  /
fnd_message.set_string('Message');
*
ERROR at line 3:
ORA-06550: line 3, column 1:
PLS-00201: identifier 'FND_MESSAGE.SET_STRING' must be declared
ORA-06550: line 3, column 1:
PL/SQL: Statement ignored
ORA-06550: line 4, column 1:
PLS-00201: identifier 'FND_MESSAGE.SHOW' must be declared
ORA-06550: line 4, column 1:
PL/SQL: Statement ignored


SQL> 

desc FND_MESSAGE;