Good day, we are still new user of Maximo at our company. I created tasks on Job plans. I completed them and closed them, on All records the tasks are list as children and they have it's unique word order number and now I can't delete them even if I try changing status. This was not in a Demo account it was on the live account.

Job Plan Tasks, become work plan tasks, and get a unique work order number on creation. This is core functionality. If you want the numbering to be different, there are instructions here for reference.