Why can’t I train anymore

The 45 minute limit is from after your encounter with the Fateweaver after you leave the tower (the starting dungeon). From there it is 45 minutes of gameplay (menus and dialog stop the clock), and you would seem to be limited to the area around the village of Gorhart for the duration of the demo. Areas not available will have the shimmering barrier there.

It is the chance that you will succeed at that dialog option (such as getting some extra money or items from the NPC).

Points in the Persuasion skill raise this chance.

I haven't run into any negative side effects with attempting and failing these sorts of checks, other than the fact that you don't get whatever bonus they would have given you, and you can't try again.