When amending an employee's sort code on their Bank page, the sort code field may be greyed out or missing if the field setting is set to Read-Only or not included.
To resolve this you will need to take a look at the field setting and ensure it's set to Optional:
Go to Setup and click Settings.
Click Field Settings and search for FormData_PersonBank_m.PrimaryBankSortCode.
Click this setting and make sure the Include box is ticked.
Select Optional from the Display dropdown.
Click Save.
Once this is done, refresh the browser page and go back to the employee's profile to amend their sort code.