SQL & PL/SQL

I need to create a directory depending on the path  ( for example it will be /export/home/'camsdocd/meterwatch for development , /export/home/'camsdocq/meterwatch for QA and/export/home/'camsdocp/meterwatch for production

Here is what I thought but I am getting the error below

declare

v_camsdoc v$instance.instance_name%type;

v_stmt varchar2(1000);

begin

select instance_name

into v_camsdoc

from v$instance;

  v_stmt :=  'CREATE OR REPLACE DIRECTORY METERWATCH AS /export/home/'||v_camsdoc||'/meterwatch';

   execute immediate v_stmt;

end;

Error at line 1

ORA-01780: string literal required

ORA-06512: at line 9