What version of PHP are you running?
Porter doesn't currently work with 7.0
See this discussion if that is the issue: https://open.vanillaforums.com/discussion/33808/vanilla-porter-and-php-7-1 (View Post)
Welcome to the community.
It is not clear what you have done.
If you have only deleted the category via the Dashboard, you should be able to restore the discussions to it if you re… (View Post)
Which tells you that there was an error accessing the Database. This could be down to a variety of issues, and really, only the web host would be able to identify what the issue was. (View Post)