Please make sure account_cancel module is installed on your cloud database. If it's installed and you have access, then you can go to accounting->configuration->Journals. Then open Journal  and under Advance Setting tab just check ""allow cancel entry", then you can unpost your journal entry and do modification.

You need to install "account_cancel" module and tick the field "Allow Cancelling Entries" on the corresponding journal. This field will be on "Advanced Settings" tab on journal form.