The tenant was either not provided in the URL or it does not exist.
In order to access the iSeeLive management portal, you need to specify the tenant that you'd like to manage.
The URL you go to should look something like this:
https://manage.iseelive.com/isee
where isee is the tenant you are trying to access.
If you require further assistance obtaining this URL, please contact an organisation administrator or iSeeLive support.