I'm trying to delete a past purchase from 2020. We did make this purchase in 2020, but it is carrying over to my 2021 reports.

Thanks for coming to the Community for your first concern, @mousebegone. 

As an insight, we can void or delete transactions made in error so it won't affect your account balance or reports. Otherwise, consult your accountant so they can help you find a better way to handle it. 

To void or delete the purchases, follow these steps below: 

1. Go to the Expenses menu.
2. Search and open the transaction, then review the transaction and decide if you need to void or delete it.
3. If you want to include the void or delete date, add a note to the Memo or Message section.
4. Click the More option at the bottom. 
5. Click Void or Delete.

Please know that deleted transactions are completely erasing from your books. It won't appear anywhere on your reports or in your accounts. 

Additionally, check out this article for more insights: Void or delete transactions in QuickBooks Online.

For future reference, use your Audit Log to recover some details in case you'll need to re-enter them. 

Come again and post some more if you have other concerns. I'm always here to be your QuickBooks guide.