Well, I don't see you passed in the parameters. Let's say you did, here I created two rules;
The first one creates password:
If Customer IS NEW AND Customer.GeneratedPWD IS UNDEFINED then Customer.GeneratedPWD= GENERATE_PWD(8,15,2,2,2)
The second, gets executed after the password was generated and gets assigned:
If Customer IS NEW AND Customer.Password IS UNDEFINED AND Customer.GeneratedPWD IS DEFINED Then Customer.Password=Customer.GeneratedPWD
Hope this helps!