I recommend try updating the apps Softaculous > Software > General scripts > Update. Sometimes that causes errors.
Also check to ensure you have the requirements for Blesta. Softaculous > Software > Script requirements.
Otherwise I recommend contacting Softaculous direct on their forum, their team are very helpful with their software. If you don't own the server try uploading the files manually, if that works then it's a issue with Softaculous.