Hello, aseel993,
Thanks for using Apple Support Communities, it's great to have you here. We can definitely help you get Family Sharing organized in the way you'd like.
In order to set a member of your Family Sharing group as a "Parent/Guardian" in that group, you will need the Organizer of your group to make that change from one of their devices. The Organizer is the person who initially set up the Family Sharing group at the time of creation. Are you the one who set up your Family Sharing group? If so, then you will simply want to follow the steps here: Approve what kids buy with Ask to Buy - Apple Support. Look for the steps in the section titled "How to choose another approver".
However, if you've done that already and it's still not working, we want to recommend that you try adding a Child or Teen to your Family Sharing group, if possible. You'd want to follow the steps here: Set up Family Sharing - Apple Support. Once you've done that, go ahead and check back in on the Adult account you mentioned earlier and see if you are able to successfully update that account to a Parent/Guardian account.
We hope that this information is helpful.
Have a great day!