Today I decided to change my Pixel 5 android version. When I tried to execute the command flash-all I got the error message saying ‘fastboot’ is not recognized as an internal or external command. After few seconds I realised I need to move all unzipped files with the new system image into the same folder where platform-tools are so the flash-all will see fastboot file. Otherwise you have to set up the access path in Windows which is pointless. So simply move the image files into the same folder as fastboot is located (usually platform-tools) and and it will work!