the disk you inserted was not readable by this computer

I'm using a MacBook Pro with OS X El Capitan Version 10.11.6. I'm trying to upgrade to Catalina 10.14 - went out and bought a new EasyStore portable hard drive to use with Time Machine. Plugged it in and got the message "The disk you inserted was not readable by this computer." What do I do now? I need to upgrade my OS in order to use my tax software before the 15th of July! Thanks in advance !