Right Click on your USB drive option in TransMac and select Format Disk for Mac.Download TransMac's latest version for Windows from this link.
There are 2 Methods to Create a Bootable USB drive of Mac OS on your Windows PC. You can download it from the Apps Store or you can find Mac OS Image at ISORIVER. Then you need a working Mac OS ISO or DMG Installation Image. To get started with you require a USB drive with at least 8GB storage (16GB recommended). Reinstalling Mac OS is a time-consuming process and requires a lot of patience that's why recommend doing this process in your free time or weekends. Macbooks or iMacs hardly require reinstallation of their Mac OS, but when they do then it's a fairly difficult process especially if your secondary computer is Windows.