Thank you for contacting us, CarolineS.
Yes, you can enter old transactions in QuickBooks Online. However, this will affect the reconciled account and the VAT return in the system.
You need to undo the reconciliation and start over after putting in other entries. I've added this article, Adjust your VAT liability for more details and instructions on how to correct your returns.
Our QuickBooks Online Technical Team can also help in making the adjustment. To contact them:
- Log in to your account.
- Click (?) Help at the top right.
- Select Contact Us to connect with a live support agent.
- Enter a brief discussion of your concern, then click Let's talk.
- Choose a way to connect with a live agent.
Let me know if there's anything else you need, and I'm always around to help you out.