Are you trying to use a voucher “now” after the class is complete, final taken, and grade posted? There is a deadline of the last day of class so you can’t use a voucher to retroactively remove a posted grade. I think it’ll be a hopeless cause to try to argue with anyone in the administration that failing to understand the system is a justification worthy of allowing a posted graded to be removed.
The system is actually pretty generous allowing a limited number of drops right up till the last day of class. You do have to talk to your advisor to initiate the process as there are limits (one per semester, 3 total, and you have to stay above 36 units after the drop). Also, like any drop/withdrawl, it makes a potentially bad grade go away but you also lose the credit so if the class is required or a pre-req for something else you’ll have to make it up. That’s part of why the advisor needs to be involved. A lot of classes after the intro/freshman classes are only offered once a year or once every two years so that has to be considered when deciding to drop a class to avoid a C or D. Obviously if you get to the last day of class and it’s clear that nothing you could do on the final will get you a passing grade then the late-drop would be a God-send removing the sure-F from the transcript.