1. Here's what it means

"No license seats are currently available" means that all the Seats of the embedded License Key in your Lumion installation are currently in use on other computer(s).

1.1: To resolve the problem, please close Lumion on the computer(s) that Lumion is running on.

1.2: If Lumion is not running on the computer that is shown in the error message, please start and quit Lumion on that computer to release the Seat. 

1.3: If that is not possible, please follow these instructions:

Knowledge Base article: How do you release a Lumion License Key?