Updating Same Record with Multiple Times at the Same Time on Oracle

I have a Voucher Management System. And I also have a campaign on that system works first click first get order. First minutes on the start of campaign, so many people trying to get to vouchers. And every time when they try to do this, the same mechanism works on thedesign image.

At the updating number of given voucher, I realize a bottleneck. All this updates trying to update same row on a limited time. Because of that, transactions adding a queue and waiting for the current update.

After the campaign, I see some updates waited for 10 seconds. How can I solve this?

Firstly, I try to minimize query execution time, but it is already a simple query.