Is this a payrun from a previous financial year? If so, this is because that financial year has been finalised.
If it is a payrun from the current financial year, then you only have the ability to edit and delete the last one.
If what you are wanting to do is unlock a payrun in order to make changes, then creating a payrun to reflect the changes you want would be a better way to do it.
Lauren Ainslie
Queanbeyan Bookkeeping