Hi Nathan,
I’m sorry you are experiencing difficulties with this.
If you have indeed run out of installations then you can manage this by visiting our Account Central > License Keys page – simply disconnect those domains that are no longer in use.
If you only have a single-site license and the domain appears to be correct then try removing your license key on your site (in Settings > Licenses) as well as via our License Keys page here at tri.be, save the changes and try re-entering.
One other consideration is that your license key may have expired. So if it has been 12 months or more since you made the purchase you would have to renew first of all.
Last but not least, you can still upgrade manually – by downloading the latest copy of The Events Calendar from wordpress.org and you can download Events Calendar PRO from Accounts Central > Downloads and update that way, should the above steps fail.
Let me know if you have further difficulties in that regard.
Barry