You're welcome.
The problem is one of your use of English as a second language and the instructions not being particularly clear. A place holder is like a picture frame, a space reserved for a/the picture, the picture might change but the place holder/picture frame doesn't change.
For example if you had a USB storage device already plugged in then the next one plugged in wouldn't be E: but F: that is why the place holder is there as it 'imports' the drive letter of the newly inserted USB storage device, so that you don't have to hard code it.
Unfortunately there is no way to remove the Cancel button as that is a part of the avast ashQuick.exe (right click, avast Quick scan), so normally this is initiated by the user to check a single file, etc. so they have to have the option to cancel a scan that 'they' initiated.
So the use of the ashQuick.exe in this way with the USBVirusScan isn't how it is meant to be used, but rather is a workaround to allow for USBs to be scanned. You can't stop someone from cancelling, so you have to educate people who use 'your' system that this scan will take place and it must be allowed to run (or they can't use your system, reasonable).
I have just done a scan of a USB containing 532MB of small files and it didn't take very long, but my system isn't slow.