Digitalam most likely your server timed out before Flarum was able to create and/or rename all the required database columns.
I believe the Flags extension should be enabled immediately following installation so this might indicate the Flarum installation itself timed out. You might have other parts of the database broken, it might be safer to try to re-install.
If it still doesn't work, you might be able to change your PHP max execution time temporarily, or run the Flarum installation on a different computer then import the database on the server.
There's also a way to install Flarum from the command line which usually has no max execution time, but I don't remember where we describe the procedure. Someone else might be able to share this.
