Invalid serial number, second install

I am trying to install Photoshop Elements 2021 on a second computer. I have it working on my desktop, but would also like to be able to use it when I travel.  I get a message "the serial number you entered is invalid".  It's not.  I checked.  Is it not allowed to have my software on two machines?  All I can find online repeatedly loops me back to purchase options and I will not buy it twice.  I don't want to waste my time on a phone call if I'm only going to be told I can't run this software on more than one machine.