I deleted your screenshot because it showed your License ID and Password. I took a look at your activation history and it says that you have one activation left and you successfully activated the license on a computer yesterday. Are you all set then? (Also, if you're concerned about your password, I can reset it on the back end for you but it may mean you have to re-activate the license on your computer after the password reset so I didn't want to do it without asking you first.)

@Juriaan Is correct about the root of the issue, you probably had the license already activated on the maximum number of computers. 

As stated on the TroikaTronix Store hosted by FastSpring

For each Buy-to-Own or Subscription license you purchase you can activate Isadora on up to two online or offline computers, though you may only use the license on one of those two computers at a time. Each Rental license you purchase allows you to activate Isadora on one computer. In all cases, you may deactivate a license on one computer in order to activate it on a different machine.

Either that or you were trying to use your Customer ID and Customer Password (only used to log into the Online License Portal) to activate the license on a computer instead of using the License ID and Activation Password. I admit that I deleted your screenshot before I thought to verify this myself, I was just concerned about keeping your license information secure.

Best wishes,