The facts are locked
Bank operations are imported from your bank and kept in sync with it. That's why their facts are locked. The app shows the note: Amount, date and account are imported from bank and cannot be edited.
How to recognise them
A bank operation carries badges that mark it as a Bank transaction, so you can tell it apart from one you entered yourself.
What you can still change
You can change its Category, Description and Tags. That's your interpretation of the operation, not the bank fact — so the app lets you edit it freely.
Why you can't delete it
You can't delete a bank operation — it would simply reappear on the next sync. If you want to stop importing, disconnect the bank instead.