There are three likely reasons why you cannot unpost a purchase invoice which are outlined below.
1. The invoice document is allocated to another document and the way to check that is. By going to the ellipse button and clicking on the Reallocate button

To be able to unpost the invoice you must remove any documents against this invoice. Below the value under the Allocated (currency) column and click Apply.

2.Or the transaction is on a VAT return that has been closed. In this scenario the only option would to be use the Reverse button the ellipse button.

3. Or the document has been matched on a bank reconciliation. Then again the only option would be to reverse the document, you cannot unpost it.
If it is not related to any of the above reasons please contact iplicit support for further assistance.