I am doing the quest tutorial from creationkit.com and tried to delete my enemy character. It says deleted in Object Window, but tried to remake it again, tried to place somewhere, and it said "Form's ID is not unique! Previous form is type Actor."... What do I do?

At a guess. try saving and reloading your data file. The content might not be actually removed until you do this.

Cipscis

Pretty much this. Although delted, the game will still "save" the data until you exit the program, or reload the plugin. While it's still "saved", you can't use the same FormID. Annoying, yes... until you accidentally delete a complex script and this feature allows you to quickly undelete it!

It worked! Thanks!