If you want to Write Off an Invoice, you must first make sure that the LogiPro user has the "Write Off Invoices" permission. This can be found via Admin > Users then selecting the User in question and click Edit (or double click on the User). Click on the Permissions tab of the User's properties window.

Expand the Accounts section and click on the Permission column header to sort the list Z-A this should bring the Write Off Invoices permission to the top. Then tick the adjacent box, click OK on the window and then OK in the User's window.

Additionally there is another setting that limits the amount per invoice that can be written off. This is accessed via Admin > General Settings > General Tab > Maximum Write Off Amount.

In order for the Write Off button to appear on an invoice, you may need to click the small button on the right of the invoice window, indicated below (1) this will make the Write Off button (2) appear. If your invoice window is large enough the Write Off button may already appear next to the Undo button.

Another factor in whether an Invoice can be written off, is that the company's accounts must not be closed after the date of the invoice.

This can be checked on a Company's Accounting tab and observing the Accounting Closure Date field.

So taking the Invoice above as an example, it can be written off because the user has the correct permission and the date of the Invoice is after the Account Closure Date.