When I try to edit or add anything on my Windows 8.1, this message pops up and I can't delete anything. Any help is welcome
The UAC message is asking for administrator approval to run this program. In this case it was cmd.exe you were running, and I assume you were trying to run it as an administrator.
If you are signed in as an administrator you could just click the 'Yes' button at this point. If you are signed in as a standard user you would be asked for the name and password of another administrator account before you could click 'Yes'.
Neither of those are happening for you, the 'Yes' button looks greyed out, meaning you cannot click it.
This means that there are no administrator accounts on this PC. Usually this is because a user has either deleted their only administrator account, or they have demoted their only user account from an administrator to a standard user. As you have found, you need at least one administrator account on a PC to be able to use it fully.
Your best option is to enable the built-in account named Administrator. Then sign in as Administrator (by default it has no password) and promote your user account back to being an administrator.
To enable the built-in Administrator you'll need to use Option Two in this tutorial.
How to Enable or Disable Built-in Elevated "Administrator" Account Windows 8 and 8.1 This tutorial will show you how to enable or disable the hidden built-in elevated Administrator account in Windows 8, Windows RT, Windows 8.1, and Windows RT 8.1. You must be signed in as an administrator to...
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When you are back to normal, create a second user account and make it an administrator so that you don't get stuck like this again. You can then disable the built-in Administrator account.