Once set up, if a negotiating body has already been assigned to employees it can no longer be deleted. This is because the employees will already have salary lines on their Pay History page that use this negotiating body and deleting it would cause issues with Payroll.
Instead of deleting a negotiating body you will need to mark it as obsolete by following the steps below:
Go to Setup, then Settings.
Click Module Settings, then Payroll Settings.
Click Pay Grade and Payscale.
Click on the title of the negotiating body.
Tick the Obsolete box and click Save.