If there is only one sales manager in the company, then this usage is correct. In this case, it is also usual to capitalise the title, as in "Sales Manager".
If there are several sales managers in the company, then it would be more normal to say "They have made me a sales manager". However, even in this case, it is not wrong to leave out the "a", as in "They promoted me to sales manager", because here you are referring to the rank, rather than the actual specific position.
Student or Learner