Why can’t I edit my unprocessed banking transaction?

It has probably been reconciled. Once an unprocessed transaction has been reconciled, that indicates that the information on the transaction was correct.
If the reconciled unprocessed banking transaction needs to be corrected, you will need to make note of the date it was reconciled. Then, go into the bank recon and enter that same date. Find the transaction. Unreconciled it. Edit it. Save, and redo the bank recon.