How can I know my username

The dialog is asking for the user name and password for "administrator" account, but your account is somehow set to "Standard" account, and there is no other administrator account as I can see in the screenshot.

So, try this to create new administrator account, then change your account back to administrator.

1. Reboot in single user mode by holding cmd-S (macOS Sierra: Start up in single-user mode)
2. Wait for prompt and type in "/sbin/mount -uw /" and hit return. This makes the boot volume writable. ( the boot volume is mounted in read-only mode on booting in single user mode )
3. Type in "rm /private/var/db/.AppleSetupDone" and hit return. This will create the indication for completion of system setup.
4. Type in "reboot" and hit return to reboot the Mac. You'll get the setup assistant running. Create new administrator account with new name in its procedure.
5. Log in with new administrator account.
6. Open System Preferences>Users & Group.
7. Click the lock icon on lower left corner and enter password for the new account. On this time, the new administrator name should be in [User name] field on the dialog.
8. Click your account and give check on "Allow user to administer this computer"
9. Close System Preference and logout.
10. Login with your account.

Now you should be able to do the task you wanted as wrote in the original post.

You can delete the administrator account just created if you confirmed the fix.

However, you can keep it for the accident in future like this case.

Look at your stored account: /HDD/Users/user_name/. You can change it but it's not that easy. Change the name of your macOS user account and home folder - Apple Support. Now, it appears your account is Standard which means you may not have authority to make the change. Then you need the Admin user to make the change. It does not appear there is an Admin account.

In your Users and Groups Systems Preferences pane (the one you were on in the photograph), your username should be the option that was highlighted in your photo to the left (I said 'right' earlier and that was incorrect 😉). Use that username when you try to sign in on the User Name in the dialog box. If that does not work, then you will probably need to add back an administrator account (see this thread) or take it into an Apple Store so that you can get assistance. I hope that helps!

In your case, from your picture, your username will be the 'Current User' username that you have highlighted to the right in the System Preferences box. That should help. It looks like your username starts with MacBook from the picture. I hope that answers your question!

Ok, the username of your account is whatever you usually use to log into your computer. What is the name at the top of the screen when you sign in? If that username doesn't work, then, as Kappy pointed out, you apparently don't have the authority to make the change since you are not the administrator of your computer.