Your Channels for Mac App is unable to connect because all of your Lightspeed Onsite Licenses are currently in use.
The Channels for Mac client automatically logs in and out of Lightspeed, just like a normal user. Most of the time Channels is reading data, but when creating invoices and customers in Lightspeed the Lightspeed API requires that a Lightspeed User License be available.
To resolve this issue log one user out of Lightspeed for approximately 10 minutes. This will allow Channels time to login and do what needs to be done.
If the issue persists, you may want to contact Lightspeed about acquiring an additional User License.
If data is deleted in Lightspeed errors will happen when that data is referenced. Those errors can lock a user license for a short time. Deleting data is strongly discouraged and will lead to integration issues. If you have recently deleted any data in Lightspeed Onsite please contact support.