While I agree with David's analysis of the shortcomings of your implementation the actual cause of that error message is this bug:
time_request < trunc(sysdate - ' || SUCCESS_PERIOD || '- 1);
SUCCESS_PERIOD
is a parameter and you presumably want to use it in your date calculation. However you have coded it as a string; a string is not a number and so we cannot use in in a substraction.
I'm not quite sure what arithmetic you're trying to achieve, but I think you want either this ...
time_request < trunc(sysdate - ( SUCCESS_PERIOD - 1));
... or this ...
time_request < trunc(sysdate - ( SUCCESS_PERIOD + 1) );
... depending on how you want to modify the value of SUCCESS_PERIOD
.