How to remove new user account created after using Windows Migration Assistant (PC to Mac)?

I've been using this Mac since 2013, now with Mojave. I wanted to transfer some files from a Windows PC to this Mac. I tried using an Ethernet connection between the two computers (1 m apart), but it didn't work.

After some research, I learn about Windows Migration Assistant on this official site. This page said it could help me transferring files but it didn't mention these important aspects of the process:

1. That I had to close my current user's session.
2. That the transferred files will be copied to a new user account.
3. The location (on this new account) of those copied files.

I need those files on my current and only user account that I've been using on this Mac from day one, not on a new account (one which I did not expect). I can't find the copied files on that new account. It doesn't matter now, please don't suggest me where I could find those files.

This is what I want now: I want to remove that new account and erase everything on it.

From my admin account, I tried: System Settings > Users & Groups > unlock changes > select the new account > click on remove (minus sign), but then a window asks me for the password of the new account.

Here's the problem: for the new account, I used the same password as my main account and it does not work. I double-checked if it was the correct password, and it is: I can log in to the new account with the correct password. But when I try to remove it (on my admin account) the window shakes as if the password was wrong (I can't modify the user's name).

I repeat: the password for the admin and the new account are the same.

Now I have a new account that I didn't want, that I don't need, that I can't remove, that it's taking space (with files I couldn't find), and lost a couple of valuable hours of my day. All this could have been avoided if Apple explained the whole migration process in detail.

Thanks for any advice.