There was an add on my computer that won’t go away I took it to Best Buy it’s fixed but why is it there and how do you get rid of it?
Thank you for using Apple Support Communities! Based on your post, there was an ad on your Mac that wouldn't go away. There are a few things you could do in the future should this happen again.
First, we'd check out how to hide ads in Safari on your Mac: Hide ads when reading articles in Safari on Mac
Note: Select the appropriate macOS version at the top of this page to see how to avoid ads while reading articles in Safari.
In case it pops up, you could block pop ups on your Mac: Block pop-ups in Safari on Mac
Note: Same as the above, select the appropriate macOS version at the top of the page.
Lastly, if you can't close the window or it continues to show up after quitting the Safari app, then you can always try to remove the app's saved state by holding down the Shift key while you launch the app (Safari). Release the key once it opens a new window and any previously opened windows should be removed.
Hope it helps out, take care!