The second line translates to “The specified path was not found.”
The script will abort the checksum checking upon any error giving the same message regardless of what kind of error actually occurred.
So it seems that the error message is not about the checksum, but some necessary path was not found before even getting to the checksum checking.
Windows as of late is by default displaying ZIP files as folders you can do some limited stuff in and with. The “Going back” guide states that you should extract the ZIP file. Did you do that, or do you operate in the still zipped folder (which wouldn’t work because the script wouldn’t find anything it needs resulting in the exact error behaviour you cite here)?
If you have unzipped the ZIP file, open a cmd, cd into the directory with all the unzipped files, then cd into bin-msys and try fastboot devices with (your phone in fastboot mode and) the fastboot.exe which is there instead of the one in your Android SDK.