Hello there, @maryqc. Welcome to the Community.
Let me share details about attaching receipts after a reconciliation in QuickBooks Online (QBO). This way, you can effectively manage your transactions and keep your account updated.
You're able to attach receipts after completing a reconciliation in QBO. When you're only attaching receipts without changing the reconciled transaction's amount, date, and/or account category, you're not affecting your reconciliation. The warning you've received only indicates that you're editing a reconciled transaction. You can ignore it as long as you aren't changing any info mentioned above. Then, attach the receipts accordingly.
To further guide you in making sure your transactions match your real-life bank statements, I'd recommend checking out this article: The reconcile workflow in QuickBooks. It includes topics about starting, fixing, and editing completed reconciliations.
Let me know in the comments below if you have other concerns about managing receipts and reconciling accounts in QBO. I'm just around to help. Take care always.