OK, if I understand it correctly, you mean you see the device USBasp in Device Manager? Is there a question mark next to it? ‘Cause if there is a question mark, that means the driver didn’t work properly.
Can you post the log file? I would need to check the exact error it reports. By the way, since you already know your device is 2.1, you don’t have to click on ‘Detect Device’, you can simply select OpenSprinkler_v2.1 from the dropdown list, and click on the Update Firmware button. The reason is that if you clicked on ‘Detect Device’ first, the device will exit bootloader and so you will have to re-enter bootloader before clicking on Update Firmware.