A Pleo expense has a category that's linked to an inactive account in Fortnox.
There are two alternative solutions:
In Pleo, change the category of the affected expenses.
Change the status of the affected account in Fortnox. To do so, click Register → Kontoplan. Click on the "Inactive" filter, click on the individual account, change the "active" status from "no" to "yes", then click "save".