enter self as main user on imac

Define "clear"?

What are you seeing on screen currently?

If it was correctly erased, it should have been at the Hello screen, ready to setup, and you can then create your own user during setup.

Did she follow the steps in the link below?

click here ➜ What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

If so, then just run through the normal setup and create your user.

click here ➜ Set up your iMac - Apple Support

If its somewhere else, then please explain what screen you are at or what you see.

Before you begin using it please do the following What to do before you sell, give away, or trade in your Mac, if you do not your Mac will be a mess.. When you say it is all clear of her that really tells us absolutely nothing, please ask her if she followed the steps in the link I provided. If she did not, it is NOT clear of her!!