I thought I'd chime in on this even though it's closed.
The restriction sounds like it was placed by Softaculous, whether or not they do this for anything else I don't know. However, if the subdomain you install Blesta under does not exist yet it is possible that your trial license would have to be re-issued. This is especially true if you access the install by IP or some other hostname/subdirectory and then later set up the subdomain.
In your case I would suggest a manual install. It's pretty straight forward, and should take no more than 15 minutes. See https://docs.blesta.com/display/user/Installing+Blesta