This will most likely be caused because you either don't have exiftool in your path, or dopus doesn't know where it is. So a button command would look like this:
cd "C:\path\to\exiftool"
@async:"C:\path\to\exiftoolexiftool.exe" @ {allfilepath|filem}
Based on plenty of info detailed in this forum, I think the simple way is to just drag the command onto the toolbar, and a helpful dialog pops up to assist in creating the command for the button.
I'd highly recommend watching this video: